Samples of the commercial catch of mackerel Scomber japonicus from Ensenada, Vizcaíno Bay, Magdalena Bay and the Gulf of California were analyzed to show biological evidence that supports a hypothesis of two stocks of mackerel. This evidence is the reproductive period, maturity cycle, deposition pattern of the otolith bands, length (L50%) and age (E50%) of reproduction and composition by length and age of the catch. Two stocks were identified, one that is distributed from southem California to Vizcaíno Bay and the other in the Gulf of California. Both stocks overlapped at Magdalena Bay, which seems be a feeding site where young individuals that have not been recruited to the spawning stock inhabit.