Maturation in the onychoteuthid squid Moroteuthis ingens was found to be irreversible, with death following shortly after sexual maturation and spawning. Both males and females were found with spent gonads. The ovary reaches very large sizes in mature females and probably prevents feeding by constricting the caecum. There was also a marked difference in the tissue integrity between immature females and females which had reached sexual maturity. Mature and spent females showed advanced tissue breakdown with individuals having a thin mantle wall with an inelastic, gelatinous appearance. Histological examination of the mantle wall revealed that the tissue breakdown was due to a drastic histolysis of muscle tissue and, to a lesser extent, collagen fibres. Mature males also showed some tissue breakdown and loss of muscle fibres but this was not as dramatic as in the females. These features are considered in relation to processes contributing to terminal maturation in M. ingens.