Three genetically distinct subpopulations of the Pacific sardine have been differentiated by the frequency of occurrence of a C-positive blood factor. Two of these subpopulations, a northern and a southern one, live off the coast of California and the outer coast of Baja California, Mexico. The third group inhabits the Gulf of California. The C-positive factor occurred in 13.6% of the northern subpopulation, 6.0% of the southern, and 16.8% of the Gulf subpopulation.