Worm-free cats about 12 weeks old were infected with 100, 500, 1,000 and 2,000 third-stage larvae (L3) of Ancylostoma tubaeforme. The changes in packed cell volume (PCV) and haemoglobin content were monitored, and the worm populations developing in each infection recorded. Cats receiving infection doses of 1,000 L3 and above showed morbidity symptoms by the third week and a rapid decline especially in the PCV value. The low dose rates (500 L3 and below) produced an apparently symptomless condition without pronounced changes in the blood values. Adult hookworm populations were linearly related to the logarithm of the dose of infective larvae administered, but there was a lower female to male ratio (FMR) at high dose levels. No significant size differences were observed in the adult hookworm populations.