In 2 yr of bacteriologic culturing of 297 California sea lions (Zalophus californianus), 154 harbor seals (Phoca vitulina), and 89 northern elephant seals (Mirounga angustirostris) stranded alive along the California coast, the most frequent isolates from inflammatory lesions in live animals were Escherichia coli, Streptococcus viridans, Listeria ivanovii, beta-hemolytic Streptococcus spp., and Enterococcus spp. This is the first report of L. ivanovii isolation from a marine mammal. The common isolates from lung and liver in animals dying during rehabilitation were E. coli, Salmonella spp., Klebsiella spp., Pseudomonas spp., Aeromonas spp., Proteus spp., and Staphylococcus aureus. The most common isolates from brain were Enterococcus spp., E. coli, Klebsiella spp., and Pseudomonas spp. Ocular lesions were seen most often in harbor seals and elephant seals, from which the isolates cultured included Pseudomonas spp., Enterococcus spp., Streptococcus viridans, E. coli, Staphylococcus aureus, Proteus spp., Morganella morganii, Moraxella spp., beta-hemolytic Streptococcus spp., and L. ivanovii. Nine different Salmonella serotypes were isolated from 49 animals; S. newport was the most common. These results should enable those working clinically with these species to make logical decisions in choosing initial antimicrobial therapy.