We have studied the protamine scombrine α from the mackerel Scomber scombrus. Scombrine ex is found phosphorylated in spermatid nuclei, but not in nuclei of ripe sperm. It is a typical fish protamine, made up of two distinct molecular species, each of 34 amino acid residues. The primary structure of the main component of scombrine α is 100% identical to scombrine γ, the nonmicroheterogeneous protamine from Scomber australasicus (11). The second component of scombrine a is a very minor molecular species that has an isoleucine instead of a valine in position 11. Nuclear sperm-specific basic proteins display an enormous interspecific variability and it is very surprising that two different species show identical protamines. In this work we suggest that evolutionary changes in primary structure of protamines are restricted by several constitutive factors, especially when protamines either lack or have a low degree of microheterogeneity.