Larvae of Simulium metallicum, S. ochraceum and S. callidum were collected from streams in the Soconusco area of Chiapas state, Mexico, during October 1975-October 1977. Collections were performed weekly using cone traps, and streams were allocated to 3 sectors according to altitude (1200-1250 m, 1000-1050 m and 800-850 m above sea level). S. metallicum was the predominant species in higher altitude streams; this species had population peaks during October-November and reduced populations during January-March (dry season) of both years. All 3 species appeared to be equally abundant in intermediate sector streams, while in the low altitude streams S. ochraceum was the dominant species in the dry season and S. callidum during the rainy season. In general, S. ochraceum and S. metallicum were found in shallow streams, and S. callidum in deeper streams. It is suggested that measures for Simulium control are applied to streams during April-May in streams located at 800-1100 m; these recommendations would be valid for neighbouring areas of Guatemala as well as the Soconusco area of Chiapas. Inglés