Culex quinquefasciatus larvae were effectively controlled by applications of Bacillus thuringiensis serotype H-14. Two road-side ditches were treated every 1-4 days from August 16 through October 14, 1979 with ca. 0.6 gm/m3 of a preparation assayed at 491 International Toxic Units per mg. A large, continuously ovipositing adult population maintained a newly hatching larval population throughout the test. A population of 4th instar and pupae developed in the untreated ditch as normally expected, but these stages were not produced in the treated ditches.