Laboratory-bred and feral chimpanzees (Pan troglodytes) and young, laboratory-bred, patas monkeys (Erythrocebus patas) exposed to infective larvae of Necator americanus cultured from human feces developed patent infections in 42 to 54 days; adult, feral patas monkeys were refractory to infec- tion. Patent infections persisted for 16 months in the chimpanzee and for 10 mos. in the monkey with- out any indication of waning. Variation in egg output among animals exposed to the same number of larvae suggested that there was considerable variation in individual susceptibility. It was also evident that large numbers of worms, in some cases, will produce severe anemia in the host.