Abstract: A flask for sterile liquids capable of supporting a terminal sterilization of pharmaceutical quality. The flask has a tamper-proof closure comprising a cap screwed on its neck and a locking ring formed in two parts axially superposed and joined by a breakable zone. One of the parts is axially carried away by the cap during opening of the flask and the other part is held axially by a shoulder formed on the neck. The cap encloses a seal made of elastomeric material inserted between its bottom and the edge of the neck, this seal being axially solid with the cap and comprising a skirt applying itself against the inner surface of the neck in a bacteriologically sealproof manner along a height sufficient to ensure a bacteriological closure seal until the locking ring is broken.