Incidence and prevalence of malaria was studied in the predominantly tribal district of Koraput, Orissa state. In the mass blood surveys in 61 malarious villages, a total of 12,122 persons were examined and 1,604 (13%) were found positive for malaria. Infant parasite rate was 23% and young children 2-4 years old were the worst affected (parasite rate 27.2%). P. falciparum, P. vivax, and P. malariae accounted for 80, 10 and 3% respectively of all infections. A sample of 2,554 children below 10 years of age were examined for splenomegaly. The spleen rate in 2-9 year old children was 22% and the average enlarged spleen (AES) was 2.06. There was no association between size of the spleen and species of parasite. Fever survey was done for a year in 22 villages and 5,511 blood samples were collected of which 24.8% were positive for malaria parasite. The annual parasite incidence (API) was 32.4 per 1000 infants and 89.5 per 1000 population. Children, 2-4 years old recorded the highest incidence (457/1000). The age pattern of prevalence and incidence indicate high level of acquired immunity in the population. Prevalence and incidence varied among different tribal communities. Prevalence of infection and splenomegaly was higher among children of Bonda, Kondh and Poroja tribes. The role of genetic factors in malaria is discussed.