Genetic diversity at loci concerned with fitness is an important part of the ability of a wild population to adapt to changes in its environment, including climatic events, disease and pollution. Research into the effects of genetic diversity on the impacts of disease on wildlife populations has focussed on genes of the major histocompatibility complex (MHC). This study investigated the genetic diversity at the MHC class II DZB gene, as well as the distribution of alleles of the same gene, for platypuses Ornithorhynchus anatinus in the Seabrook Creek Catchment in northwest Tasmania. This study detected 10 previously identified alleles and two previously unreported alleles at the MHC Class II DZB locus in 18 platypuses from the Seabrook Creek Catchment. An additional sequence isolated from two individuals was consistent with a pseudogene. Alleles were reasonably well distributed geographically through the catchment, but there was evidence of a degree of isolation at one site. Consistent with evidence that smaller wildlife populations have relatively low genetic diversity, and that there is relatively slow gene flow between river catchments, the observed genetic diversity at the MHC Class II locus was lower than those in larger previously studied river catchments but higher than those in two island populations. Consequently, this population of platypuses may have a limited capacity to respond to new infectious challenges, such as the fungal disease mucormycosis.