1)A lethal mutant: (S: 9), which was studied by EDE (1956) for its embryonic lethality, has been studied for its pupal phenotype.2)Larvae fail to contract at the end of the third instar and give rise to very elongated puparia.3)Prepupae and pupae fail to contract and this leads to the persistence of the air bubble throughout the pupal life, and to considerable damage to the appendages.4)The action of the lethal factor has been discussed.