Group A rotaviruses infect and cause diarrhea in the young of a broad range of terrestrial mammals, but it is unknown, to our knowledge, whether they infect marine mammals. During February and March of 2002 and 2003, we collected 125 serum samples and 18 rectal swab samples from Galapagos sea lion pups (GSL, Zalophus wollebaeki), and 22 serum samples from Galapagos fur seal pups (GFS, Arctocephalus galapagoensis) from nine islands of the Galapagos archipelago, Ecuador. Sera were tested for antibodies (immunoglobulin G [IgG]) to rotavirus by an enzyme immunoassay using rhesus rotavirus as the capture antigen. In addition, rectal swabs were analyzed for the presence of rotavirus genomic double-stranded RNA by silver-stained polyacrylamide gel electrophoresis. Antibodies to rotavirus were detected in 27 GSL pups (22%) and five GFS pups (23%), and rotavirus RNA was detected in the fecal sample from one GSL pup (6%). These results provide the first evidence that rotavirus infections are prevalent at an early age in Galapagos sea lions and Galapagos fur seals.