We have used acetic acid-urea-triton (AUT) gel electrophoresis and ionic exchange chromatography in order to analyze the interspecific variability and microheterogeneity pattern of the protamine-like (PL) proteins of the sperm of 4 different species of the bivalve mollusc, Mytilus. We have found that based upon these 2 criteria, it is possible to unambiguously distinguish each species from the rest. We have thus been able to corroborate the identity of M. trossulus. We have also analyzed the amino acid composition of some of the PL components for each different species. In the case of the PL-II* fraction, we have analyzed the composition of its major protein subcomponents.