The sex ratios of 4 geographically separated populations of brush-tailed possums sampled by trapping or poisoning with cyanide paste were generally imbalanced towards males. An examination of the absolute sex ratio in 1 population indicated such biases may not be real. Age distributions of sampled and absolute populations also differed, with most samples containing few old males. Sex and age biases probably result from the sampling procedures, with animal mobility influencing each animal's susceptibility to trapping or poisoning. As males are more mobile than females of similar age, the former are generally taken more frequently.