Sperm chromatin of Murex brandaris (a neogastropod mollusc) undergoes a series of structural transitions during spermiogenesis. The DNA-interacting proteins responsible for these changes as well as the mature protamines present in the ripe sperm nucleus have been characterized. The results reveal that spermiogenic nuclear proteins are protamine precursors that are subjected to a substantial number of small N-terminal deletions that gradually modify their overall charge. The composition of mature protamines is remarkably simple in turn, promoting an efficient and extremely tight packaging of DNA. The pattern of spermiogenic chromatin condensation in M. brandaris clearly departs from that corresponding to vertebrate chromatin.