House mice (Mus musculus) and harvest mice (Reithrodontomys megalotis) living in the salt marsh areas of northern California were studied in the field and in the laboratory for a one-year period. The results of 23 trapping periods indicated that house mice were 5.6 times as abundant as harvest mice. Physiological comparisons showed that house mice weighed more, ate and drank more than harvest mice, but had an equal tolerance to water deprivation. Behavioral studies demonstrated that house mice were more active, but both species had similar activity cycles; also both species behaved similarly when caged alone or when both species were caged together. Complete compatibility between species was demonstrated in captivity and there were indications of a cospecies social hierarchy which suggested that in the field a high degree of compatibility existed between species.