ObjectiveS: (a) To determine the natural infection rate of Bulinus truncatus and Biomphalaria pfeifferi snails with trematodes' cercariae. (b) To determine the emergence and rhythmicity of cercariae. (c) To elucidate the high-risk time for man and other animals to acquire infection. Methods: Snails were collected from Dawar El Mahadi Agricultural Scheme, Khartoum State, identified in the laboratory, kept at room temperature and fed on lettuce. The snails were screened weekly for six weeks for natural infection and infected snails were kept in the dark. The swimming patterns and resting position of the freshly emerged cercariae were studied using a stereomicroscope. The rhythmicity of the different types of cercariae was studied by screening three sets of 5 naturally infected snails under fluorescent light from 07.00 to 9.00 and similar sets from 9.00 to 07.00. Results: Out of ,257 screened Buli nus truncatus, 87 (4.9%) shed four types of cercariae. The highest prevalence of natural infection (9.5%) was by schistosome cercariae followed by amphistome (2.5%), xiphidiocercariae (2.4%) and lastly by avian cercariae (0.5%). However, out of 200 screened B. pfeifferi, 22 (%) shed only xiphidiocercariae. The rhythmicity studies showed that the emergence of schistosome cercariae increased steadily from 07.00 to reach its peak at .00-3.00. The emergence rhythms of avian cercariae are similar to those of the schistosome, but with an early peak at 09.00-.00. The xiphidiocercariae and amphistome cercariae started with high rate of emergence at 07.00. and decreased gradually to very low levels or complete disappearance, respectively, around sunset. Conclusion: Information on cercarial rhythmicity and chronobiological characteristics are thought to be useful in avoiding water contact during high-risk time of infection and may be helpful in the identification of closely related species and strains of cercariae.