Abstract: A cap for a flask includes a top portion; an inner collar extending downwardly from the top portion for insertion into the mouth of the flask; an outer collar spaced from the inner collar and extending downwardly from the top portion for covering the lip of the flask; detent means on the inner surface of the outer collar extending inwardly to positively engage the lip of the flask and sealingly engage the cap with the mouth of the flask; and a permeable section in the top portion within said inner collar for enabling exchange of gases in the flask.