An attempt was made to estimate the prevalence of hydatid disease amongst the Maasai people of northern Tanzania and to examine the factors which would contribute to the transmission of the disease in this region. Retrospective review of hospital records revealed an annual surgical rate of hydatid disease for 1977 through 1986 of 11 per 100,000 people. Ten (1%) of 959 people screened by ultrasound scanning had hydatid cyst-like echoes, giving a prevalence estimation of 1.4%. Thirty per cent of positive ultrasound cases were also positive in an ELISA antibody test. Three of four dogs necropsied and two of six dogs purged were found to harbour patent Echinococcus infections. Quantitative man:dog contact studies, examination of soil and water samples for Echinococcus eggs and comparison of the anti-Echinococcus antibody responders revealed that there are few marked differences in exposure to infection between the Maasai and the Turkana, who have a much higher prevalence of hydatidosis and perhaps the highest prevalence in the world. Hypotheses to explain the differences in the infection rate between these two groups of pastoralists who share a similar traditional lifestyle are presented. The Maasai, like the Turkana, had no knowledge of hydatid disease, and it is suggested that the best way to control the disease in Maasailand would be by proven dog control measures, supported by a suitable, more durable, educational programme.