Severe outbreaks of malaria occurred in the coastal villages of the Candolim Primary Health Centre (PHC) of Goa, India, in 1993 and 1994. These outbreaks were associated with accelerated construction activity with an influx of migrant laborers. The weekly application of Bacillus thuringiensis var. israelensis (B.t.i.) strain 164 at 1 g/m2 and introduction of the indigenous larvivorous fish Aplocheilus blocki in major breeding habitats of Anopheles stephensi replaced ongoing DDT spraying and pyrethrum fogging in June 1994. The objective of this study was to evaluate the effects of B.t.i. and larvivorous fish on An. stephensi and subsequent transmission of malaria in the Candolim PHC, Goa, India. In 1995 the populations of an. stephensi in larger habitats (habitats with immatures: t = 5.19, P = 0.0017; immature density: t = 3.57, P = 0.007) and smaller habitats (habitats with immature: t = 3.86, P = 0.005; immature density: t = 4.93, P = 0.002) and malaria incidence declined substantially (malaria cases: chi 2 = 712, P < 0.001; slide positivity rate: chi 2 = 10.36, P < 0.001; annual parasite index; chi 2 = 15.1, P < 0.001), whereas the incidence of malaria continued to increase in other nearby towns.