diff --git a/fastcore/script.py b/fastcore/script.py
index 67b075ae..fc221f7d 100644
--- a/fastcore/script.py
+++ b/fastcore/script.py
@@ -51,7 +51,7 @@ def main(msg:Annotated[str, "The message"],
 
 ## Short flags
 
-A capital letter in a parameter name declares a short flag: the capitalized letter becomes the short spelling and the lowercased name the long one, so `Resume:int=None` gets both `-r` and `--resume`. The capital can be any letter (`sUggest:str=None` gives `-u/--suggest`), only the first capital counts, and names without capitals get a long flag only. Since the flags are lowercased, the parameter's actual name keeps its capital -- so a Python caller writes `main(Resume=3)`, which usefully advertises that it's invoking a CLI entry point. Positional (default-less) parameters have no flags, so capitals there are left alone.
+A capital letter in a parameter name declares a short flag: the capitalized letter becomes the short spelling and the lowercased name the long one, so `Resume:int=None` gets both `-r` and `--resume`. The capital can be any letter (`sUggest:str=None` gives `-u/--suggest`), only the first capital counts, and names without capitals get a long flag only. Underscores in optional parameter names appear as hyphens (`cache_dir` becomes `--cache-dir`), while the Python argument keeps its underscore. Since the flags are lowercased, the parameter's actual name keeps its capital -- so a Python caller writes `main(Resume=3)`, which usefully advertises that it's invoking a CLI entry point. Positional (default-less) parameters have no flags, so capitals there are left alone.
 
 ## Positional params
 
@@ -141,7 +141,7 @@ def _arg_kw(k, anno, doc, default, extra, mv=None):
         else: action,d = 'store_true',False
     if action=='version':
         if 'version' not in extra and d is not inspect.Parameter.empty: extra['version'] = d
-        return f'--{k}', {'help':doc or '', **extra}
+        return f'--{k.replace("_", "-")}', {'help':doc or '', **extra}
     kw = {}
     if action and 'action' not in extra: kw['action'] = action
     if anno is not None:
@@ -154,6 +154,7 @@ def _arg_kw(k, anno, doc, default, extra, mv=None):
     kw['help'] = (doc or '') + (f" (default: {dshow})" if 'default' in kw else '')
     if negated: kw['dest'] = k
     name = f'no-{k}' if negated else k
+    if opt: name = name.replace('_', '-')
     short = first(c for c in k if c.isupper()) if opt else None
     if short is None: return f"{'--' if opt else ''}{name}", {**kw, **extra}
     kw['dest'] = k  # flags are lowercased, so argparse's derived dest would drop the capital
diff --git a/nbs/06_script.ipynb b/nbs/06_script.ipynb
index 07522680..c896ec15 100644
--- a/nbs/06_script.ipynb
+++ b/nbs/06_script.ipynb
@@ -140,7 +140,7 @@
     "#| export\n",
     "## Short flags\n",
     "\n",
-    "A capital letter in a parameter name declares a short flag: the capitalized letter becomes the short spelling and the lowercased name the long one, so `Resume:int=None` gets both `-r` and `--resume`. The capital can be any letter (`sUggest:str=None` gives `-u/--suggest`), only the first capital counts, and names without capitals get a long flag only. Since the flags are lowercased, the parameter's actual name keeps its capital -- so a Python caller writes `main(Resume=3)`, which usefully advertises that it's invoking a CLI entry point. Positional (default-less) parameters have no flags, so capitals there are left alone."
+    "A capital letter in a parameter name declares a short flag: the capitalized letter becomes the short spelling and the lowercased name the long one, so `Resume:int=None` gets both `-r` and `--resume`. The capital can be any letter (`sUggest:str=None` gives `-u/--suggest`), only the first capital counts, and names without capitals get a long flag only. Underscores in optional parameter names appear as hyphens (`cache_dir` becomes `--cache-dir`), while the Python argument keeps its underscore. Since the flags are lowercased, the parameter's actual name keeps its capital -- so a Python caller writes `main(Resume=3)`, which usefully advertises that it's invoking a CLI entry point. Positional (default-less) parameters have no flags, so capitals there are left alone."
    ]
   },
   {
@@ -295,7 +295,7 @@
     "        else: action,d = 'store_true',False\n",
     "    if action=='version':\n",
     "        if 'version' not in extra and d is not inspect.Parameter.empty: extra['version'] = d\n",
-    "        return f'--{k}', {'help':doc or '', **extra}\n",
+    "        return f'--{k.replace(\"_\", \"-\")}', {'help':doc or '', **extra}\n",
     "    kw = {}\n",
     "    if action and 'action' not in extra: kw['action'] = action\n",
     "    if anno is not None:\n",
@@ -308,6 +308,7 @@
     "    kw['help'] = (doc or '') + (f\" (default: {dshow})\" if 'default' in kw else '')\n",
     "    if negated: kw['dest'] = k\n",
     "    name = f'no-{k}' if negated else k\n",
+    "    if opt: name = name.replace('_', '-')\n",
     "    short = first(c for c in k if c.isupper()) if opt else None\n",
     "    if short is None: return f\"{'--' if opt else ''}{name}\", {**kw, **extra}\n",
     "    kw['dest'] = k  # flags are lowercased, so argparse's derived dest would drop the capital\n",
@@ -371,9 +372,9 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "test_eq(_arg_kw('a', int, 'help', 1, {}), ('--a', dict(type=int, default=1, help='help (default: 1)')))\n",
+    "test_eq(_arg_kw('some_flag', int, 'help', 1, {}), ('--some-flag', dict(type=int, default=1, help='help (default: 1)')))\n",
     "test_eq(_arg_kw('a', int, 'help', inspect.Parameter.empty, {}), ('a', dict(type=int, help='help')))\n",
-    "test_eq(_arg_kw('up', bool, 'upper?', True, {}), ('--no-up', dict(action='store_false', default=True, help='upper? (default: True)', dest='up')))\n",
+    "test_eq(_arg_kw('some_flag', bool, 'enabled?', True, {}), ('--no-some-flag', dict(action='store_false', default=True, help='enabled? (default: True)', dest='some_flag')))\n",
     "test_eq(_arg_kw('p', str, 'path', inspect.Parameter.empty, dict(opt=False, nargs='?', default='-')),\n",
     "        ('p', dict(type=str, default='-', help='path (default: -)', nargs='?')))\n",
     "test_eq(_arg_kw('a', int, 'help', 1, {}, 'int'), ('--a', dict(type=int, default=1, help='help (default: 1)', metavar='(int)')))\n",