Populations of a mosaic-tailed rat (Melomys sp.) were studied for one year in vine-thicket and mangrove habitats in the Mitchell Plateau area of the tropical Kimberley district of Western Australia. Demographic and physiological (haematological values, androgens, corticosteroids and their binding proteins) parameters were measured together with parasite prevalence and electrophoretic protein variation. Seasonal variation occurred in most parameters measured. Higher values were found in the vine-thicket for number of sympatric mammal species, growth rate of immatures and androgens. Higher values were found in the mangroves for population density, survival, size at sexual maturity, body weight, home range overlap, albumins, total and free-corticosteroids and parasite prevalence. The phenotypic frequencies of two protein polymorphisms (albumin and 6–phosphogluconate dehydrogenase) differed between sites, while a third (adenosine deaminase) showed no between-site differences; these suggest differential selection between the adjacent habitats which are separated by 10 km. The overall physiological response of Melomys was more entrained to seasonal variation in the vine-thicket than in the mangroves, which is in accord with the relative variation in the vegetation and temperature in the two areas. It is hypothesized that the greater population density in the mangroves results in behavioural feedback through the hypothalamo-pituitary-adrenal axis. This results in increased stress (higher free-corticosteroid levels) which slows the growth rate and the onset of sexual maturity. The study is the first to indicate that such a mechanism, commonly proposed for temperate small mammals, may operate in tropical areas.