6 pages, 4 figures, 2 tables 1. 1. We have characterized for the first time the major sperm-specific nuclear proteins (X, P1 and P2) of the tunicate Styela plicata. Both P1 and P2 have an amino acid composition that allows us to classify them as protamine-like proteins. 2. 2. The protein P1 of lower electrophoretic mobility has a trypsin-resistant core which is compositionally related to that of histones of the H1 family and to the PL-I protein found in the sperm of marine invertebrates. The evolutionary significance of this finding is discussed. 3. 3. In addition to P1 and P2, the sperm nucleus of S. plicata contains a protein X component which is also compositionally related to PL-I proteins from bivalve molluscs. 4. 4. Besides these sperm-specific proteins, a full complement of somatic-like histones, including a somatic-like histone H1, is also present. These histones represent only a small fraction of the total nuclear proteins of the sperm. © 1992