Efficacy of water source treatment with temephos 200 EC had been tested with three methods after treatment of a natural reservoir in a temporary river bed. 1. Residual pesticide in water treatment was measured with a biological test based on the pesticide toxicity on mosquito larvae from a laboratory stock for which the standard LD50 for the considered pesticide is stable and accurately known. After pesticide treatment, samples of the treated source water were taken. 30 larvae were placed in 200 ml of the treated water either pure or diluted in untreated water (ten dilutions tested). The mortality rate of larvae after 24 hours was observed and marked on a probit scale. The LD50 was read directly from the curve. The corresponding dilution of the water sample that contained one standard LD50 was expressed in mg.l-1, using the value of the standard LD50. This method was found reproducible and sensitive (up to 0.001 p.p.m.), depending on the value of the standard LD50. 2. Lethality of cyclopides was measured by numeration of survivals in 10 liters samples and compared to the natural population of a close untreated water reservoir in the higher part of the same river. Results showed that half-life of temephos was shorter than 3 days and the concentration corresponding to the LD50 for cyclopides was reached a week after treatment. Two or three weeks after treatment the cyclopide population was significant and further pesticide application was needed. 3. Decrease of incidence of dracunculiasis was compiled by weekly examination of the population during the season of transmission. Authors observed an incidence of 37% during the 86/87 season.(ABSTRACT TRUNCATED AT 250 WORDS)