Fleas were collected and identified for one year from dead-trapped rats and wild rodents found coexisting in several localities of the San Francisco Bay region. Interchange of flea species between host species was indicated by the following principal atypical flea-host relationships: N. fasciatus on 18.5 per cent of M. californicus and on 12.4 per cent of P. maniculatus; M. telchinum on 5.8 per cent of R. norvegicus. These 2 species plus C. wymani occurred on all principal mammal species taken. Hystrichopsylla sp., A. multidentatus and M. w. ophidius occurred with little discrimination of M. californicus and P. maniculatus but were not found on rats. Species indicated as quite host specific were O. k. nesiotus, X. cheopis, D. montanus and H. anomalus. Flea interchanges involving C. beecheyi were infrequent. P. pestis was isolated from 4 flea pools collected from one of the areas.