Abstract: A combination of laboratory flask (2) and a cap (1) adapted for closure of said flask (2), wherein said laboratory flask (2) comprises a chamber and a neck (3) connected to said chamber and giving access to the interior of said chamber. Said cap (1) is made of a deformable elastic material and is adapted for being inserted on said neck (3) by a translation movement along said neck (3), the exterior surface of said neck (3) and the interior surface of said cap (1) being provided with at least two pairs of respective cooperating means (11-12, 11-12) arranged for producing a latching effect when the cap (1) is not deformed and an unlatching effect when the cap (1) is deformed, whereby the cap (1) can be inserted on or removed from said flask (2) by exerting a force on the cap (1) but not on the flask (2). The cap is preferably a snap-on cap, with a venting position.