Of 356 samples of human sera collected from native patients in two distinct zones of Somalia, 53% were positive (greater than or equal to 1:8) to the dye-test for Toxoplasma gondii antibodies. A significantly lower incidence (P less than 0.01) of infection was found in patients living in the arid Mogadishu area, compared to that in patients from villages on the river borders, situated in humid soil zones. Furthermore, in 4.2% of the river area cases the titres were over 300 I.U., indicating that acute T. gondii infection exists among the Somalian population. These differences were regarded as being due to climatic and geographical conditions rather than to diet or socio-economic conditions.