The course of primary infection with a hamster-adapted strain of Ancylostoma ceylanicum was studied in inbred DSN and randomly bred WO/GD and WO/CR hamsters. Infective larvae were administered orally and began to develop in the small intestine without embarking on a tissue migration. Only the occasional larva was detected in other organ sites. It was concluded that the developing larvae moulted on days 3-4 and again to the pre-adult stage about 9-11 days post-infection. Worm burdens in infected hamsters were stable for at least 11 weeks after infection. There was no sudden expulsive phase and some adult worms survived for over 200 days. Overall the sex ratio of worms in groups of hamsters killed concurrently was about 50% although occasionally the ratio was biased in favour of one sex in individual animals. The blood packed cell volume (PCV) was significantly depressed 2 weeks following infection and continued to decline until a point of stability was achieved 4-5 weeks post-infection. The PCV subsequently remained depressed throughout the period of observation. Infected hamsters lost weight if kept in groups, but not when housed in separate cages. Groups of animals which lost weight did not recover to normal values within 11 weeks of infection. It is suggested that this model of hookworm infection has scope for exploring aspects of the host-parasite relationship which the canine models cannot fulfill adequately.