Cuttlefish spermiogenesis is characterized by a two-step nuclear protein transition: histones-->spermatid-specific protein (protein T)-->sperm protamine (protein Sp). A similar situation can be observed in another Cephalopod species, the squid Loligo pealeii. The protein T from Loligo consists of two structural variants, T1 and T2 (molecular masses: 10788 and 10791 Da respectively), phosphorylated to different degrees (2-6 phosphate groups). The primary structures of these two variants and of the protamine variant Sp2 were established from sequence analysis and mass spectrometric data of the proteins and their fragments. T1 and T2 are closely related proteins of 79 residues. The complete structural identity of the C-terminal domain (residues 22-79) of protein T2 with the sperm protamine Sp2 (molecular mass 8562 Da, 58 residues) strongly suggests that the testis-specific protein T2 is indeed the precursor of the protamine. The transition between the precursor protein T and protein Sp results from a hydrolytic cleavage similar to that found in many proteins that are synthesized as precursors. The processing mechanism involves the specific cleavage of a Gly-Arg bond in the sequence Met/Leu18-Lys-Gly-Gly-Arg-Arg23. Furthermore, the study provides molecular evidence on the taxonomic relationship between Loligo and Sepia.