The state of the S. haematobium infection in North Cameroon seemed sufficiently well known to regard as negligible the risks involved in planning hydro-agricultural plants. However, a series of field-studies conducted by the authors in one of these sites (SEMRY II, North of Yagoua, in the Mayo Danaï area) has shown that the spread of this disease varies widely from village to village. It is sometimes absent, and on average, it rarely exceeds 15% of the sample (approximately 4,000 people, spread over 28 survey areas). This result is obtained using a representative sample of about 70,000 people, directly or indirectly affected by the hydro-agricultural project. The technique used was a simple centrifugation, and no egg count was made. Two malacological investigations performed in the same area complemented the parasitological inquiry. Comparison of these results with previous works shows that the latter used too few observations, the statistical representativity of which was never assured. On the other hand, recent evaluations, done independently of those reported in this paper, but which are limited to a few villages, confirm that the present infection rate for S. haematobium is appreciably lower than what is believed, and that it varies considerably from village to village. This re-evaluation of the problem leads us to conclude that the hydroagricultural plants, which are intended to develop irrigated rice-growing, might cause a considerable upsurge of the disease; and that this risk should be prevented by setting up specific control programs.