diff --git a/flopy/mfusg/mfusg.py b/flopy/mfusg/mfusg.py
index b7817240e..db79228e0 100644
--- a/flopy/mfusg/mfusg.py
+++ b/flopy/mfusg/mfusg.py
@@ -535,7 +535,7 @@ def _send_load_messages(model, files_successfully_loaded, files_not_loaded):
                     print(f"      {os.path.basename(fname)}")
 
 
-def fmt_string(array):
+def fmt_string(array, free=False):
     """
     Returns a C-style fmt string for numpy savetxt that corresponds to
     the dtype.
@@ -543,14 +543,21 @@ def fmt_string(array):
     Parameters
     ----------
     array : numpy array
+    free : bool, optional
+        If True, use high-precision format (%16.9G) for floats which requires
+        FREE format in the BAS file. If False (default), use fixed 10-character
+        format (%10.2e) compatible with MODFLOW-USG's fixed-width input format.
     """
     fmts = []
+    # When FREE format is enabled in BAS, we can use high-precision output.
+    # Without FREE, MODFLOW-USG expects 10-character fixed-width fields.
+    float_fmt = "%16.9G" if free else "%10.2e"
     for field in array.dtype.descr:
         vtype = field[1][1].lower()
         if vtype in {"i", "b"}:
             fmts.append("%10d")
         elif vtype == "f":
-            fmts.append("%10.2e")
+            fmts.append(float_fmt)
         elif vtype == "o":
             fmts.append("%10s")
         elif vtype == "s":
diff --git a/flopy/mfusg/mfusgcln.py b/flopy/mfusg/mfusgcln.py
index 15741bb6f..d2e87798c 100644
--- a/flopy/mfusg/mfusgcln.py
+++ b/flopy/mfusg/mfusgcln.py
@@ -543,18 +543,23 @@ def write_file(self, f=None, check=False):
             f_cln.write(self.iac_cln.get_file_entry())
             f_cln.write(self.ja_cln.get_file_entry())
 
-        np.savetxt(f_cln, self.node_prop, fmt=fmt_string(self.node_prop), delimiter="")
+        free = self.parent.free_format_input
+        np.savetxt(
+            f_cln, self.node_prop, fmt=fmt_string(self.node_prop, free), delimiter=""
+        )
 
-        np.savetxt(f_cln, self.cln_gwc, fmt=fmt_string(self.cln_gwc), delimiter="")
+        np.savetxt(
+            f_cln, self.cln_gwc, fmt=fmt_string(self.cln_gwc, free), delimiter=""
+        )
 
         if self.nconduityp > 0:
             np.savetxt(
-                f_cln, self.cln_circ, fmt=fmt_string(self.cln_circ), delimiter=""
+                f_cln, self.cln_circ, fmt=fmt_string(self.cln_circ, free), delimiter=""
             )
 
         if self.nrectyp > 0:
             np.savetxt(
-                f_cln, self.cln_rect, fmt=fmt_string(self.cln_rect), delimiter=""
+                f_cln, self.cln_rect, fmt=fmt_string(self.cln_rect, free), delimiter=""
             )
 
         f_cln.write(self.ibound.get_file_entry())