From August 1995 to August 1997, six of 18 common dolphins (Delphinus delphis) that stranded along beaches of southern California (USA) tested antibody positive for dolphin morbillivirus (DMV). Titers ascertained by virus neutralization ranged from 1:50 to 1:910 while those determined by ELISA ranged from 1:80 to 1:195. The first individual to strand survived and was released back into the Pacific Ocean 14 mo later. Histopathologic examination of tissues from the other five dolphins did not reveal lesions characteristic of morbilliviral disease; however, morbilliviral RNA was detected in three of the five by reverse transcriptase-polymerase chain reaction testing. This is the first report of morbilliviral infection in any marine mammal species in the northern hemisphere of the Pacific Ocean. These data indicate that DMV, or a closely related morbillivirus, is present in the Pacific Ocean and infection of common dolphins may not be associated with morbillivirus disease.