The marsh deer Blastocerus dichotomus is one of the largest mammals in South America. It is classified as vulnerable by IUCN (1990) and is listed on Appendix I to the Convention on International Trade in Endangered Species of Wild Flora and Fauna (CITES). Very little field-work has been done to ascertain its status and current distribution and the survey described here is the first attempt to census the marsh deer numbers in the Iberd Natural Reserve, the Argentine stronghold of the species. The survey found only 1100 individuals with a smaller percentage of yearlings than would be expected in a healthy population. Hunting, competition with domestic stock for grazing and transmission of diseases from cattle may be combining to limit population growth and further research is needed to form the basis of a mangement plan for the species.