A mark-recapture study of filesnakes (Acrochordus arafurae) in freshwater billabongs of northern Australia provided data on >3400 snakes over a four-year period. Population densities were estimated by a modified jackknife technique, and were much higher than in most previously-studied species of snakes, with >400 snakes per hectare surface area (>100 kg ha-1) in the main study billabong. This high biomass can be maintained because of the high abundance of prey (fishes), coupled with the low metabolic requirements (and hence, low feeding rates) of acrochordid snakes. Comparisons of sex ratios in trapped samples and in population estimates showed that male filesnakes were less 'catchable' in our fyke-nets than were females. Sex ratios differed among billabongs, with a trend for more males in broad shallow back-flow billabongs and more females in narrow deep main-channel billabongs. This sex difference mirrors a size-related difference in habitat selection documented in earlier studies of this species. Filesnake populations contain a high proportion of immature snakes, because of delayed maturation in this species, but mortality rates among adult animals (especially, adult females) seem to be low. The size structure (and hence, we infer, age structure) of the population varied considerably from year to year, depending on levels of reproduction and thus, juvenile recruitment. This episodic recruitment means that populations are dominated by particular age classes, and are unlikely to attain stable age distributions.