Fifty-one diseased California sea lions, Zalophus californianus, that stranded on southern California beaches were examined by necropsy. Pneumonia and mucoid bronchiolar obstruction, secondary to infestation with the lungworm Parafilaroides decorus, were the predominant lesions but other findings are presented. Bacterial isolations and identifications were attempted when indicated and antibiotic sensitivities were noted. Comments are included on the diagnosis and treatment of the more commonly found diseases.