The recent emergence of a mixed focus of Schistosoma haematobium-Schistosoma mansoni, in the lower delta of the Senegal river, requires adapted control programmes. A mass treatment with praziquantel was organised in April 1994 by local authorities in three villages where populations had been examined. A total of 2042 subjects participated. In Savoigne S. haematobium prevailed (53% for prevalence), in Diagambaly S. haematobium (64%) and S. mansoni (76%) were both abundant, and in Boundoum S. mansoni prevailed (53%). Therapeutic coverage (80%) was assessed on a representative sample. A cohort of 968 treated subjects were followed-up 40, 100, 200 and 300 days after treatment. Six weeks after treatment, the average of egg excretion decreased by 95% for S. haematobium, ranging from 23 to one egg(s)/10 ml at Savoigne and from 14 to one egg(s)/10 ml at Diagambal. Conversely, egg excretion only decreased by 75% for S. mansoni, from 23 to six eggs/g at Boundoum and from 69 to 16 eggs/g at Diagambal, showing evidence of the low susceptibility of S. mansoni local strain to praziquantel. Ten months after treatment, reinfections with S. haematobium remained weak at Savoigne (two eggs/10 ml) while those with S. mansoni were so high at Boundoum (24 eggs/g) that they compensated the reduction of load induced by the treatment. At Diagambal, where the two parasites were present before treatment, the disappearance of the urinary schistosomiasis after treatment concurred with a dramatic increase of intestinal schistosomiasis. S. manoni egg excretion was seven times higher than before treatment (478 eggs/g). These different effects of treatment are discussed according to the ecology of transmission in the three villages.