This study was carried out to determine the importance of small field irrigation canals (abu eshreens)-of which there are over 22 000 in the Gezira Scheme- in the transmission of schistosomiasis. The observations were made during a 12-month period, November 1976-November 1977. The results indicated that during the 4-year crop rotation the abu eshreens supported snail populations in only two seasons, when they were irrigating crops of either, cotton or groundnuts. Thus, when agricultural labourers built temporary dwellings on the abu eshreen banks during these periods, the probability of finding infected snails was high. The maximum risk periods occurred during vegetable harvesting (including groundnuts) (October-December) and in the cotton picking season (January-April). Certain special situations lead to some abu eshreens playing an even greater and more predictable role in transmission. These occur when abu eshreens: (a) irrigate eucalyptus or fruit trees; (b) irrigate a block inspector's garden; or (c) are situated very close to permanent villages. Possible control measures to reduce the transmission potential of abu eshreens are discussed. These include improved maintenance and the use of either focally sprayed or slow-release molluscicides.