You can freeze mushrooms, but it will take a little extra effort on your part! Do not just throw them in the freezer and hope for the best, instead try following these instructions from cooks.com:
Freezing Raw (Freeze whole, for use within 3 months) :
Choose very fresh mushrooms; wipe with a clean cloth to remove dirt. Place each a little apart on a baking tray and open-freeze as quickly as possible. When mushrooms are frozen, pack into containers, fit on a lid and replace in freezer. Use within 3 months. Use straight from the freezer for pies, casseroles or stews or thaw for about 1 hour and use as required.
Freezing Sauteed Mushrooms:
Saute 1 lb. of mushrooms in butter with salt and pepper, using high heat for 1 or 2 minutes, stirring constantly. Cool, then store in small containers and freeze. Use frozen, or thaw to room temperature about one hour; use for omelettes, sauces or served on toast