The field trial of larviciding S. ochraceum was carried out from March-May 1979, in the Lavaderos River Valley, Municipality of San Vicente Pacaya, Department of Escuintla, Guatemala. A 10% temephos solid formulation was applied at a rate of 1 ppm for 60 min with a 14 day cycle. In the 57 sites for application, a total of 245 g of solid larvicide was applied. The larval density of the blackfly decreased rapidly in many larvicided tributaries of the river. After a few applications the density of adult females captured with A human bait for 3 h decreased to the minimum of 6 flies in comparison with 20-500 at pre-application. The results reveal the feasibility of the solid formulation of temephos against S. ochraceum in the field if the pre-application survey on the breeding streams of the target species are made. Inglés