Alpha-terthiophene (alpha-T) and erythrosin-B, the naturally occurring plant secondary metabolites, were tried for their phototoxic properties against Anopheles and Culex larvae under dark, ordinary tube light (1.9-2.4 w/m2) and sun light (680-840 w/m2). LC50 values of alpha-T for Anopheles larvae (4th instar) were found to be 154, 92 and 11 ppb under dark, tube light and sunlight, respectively. For Culex larvae corresponding LC50 values under different light conditions were 129, 97 and 22 ppb. Erythrosin-B under all photoregimens was found to be less toxic to larvae of both Anopheles and Culex sps. Also, the susceptibility of the mosquito species decreased with age, towards alpha-T and erythrosin-B. Cumulative effects in terms of delay in metamorphosis were also observed among survivors of such exposures. The effects of these compounds were also seen on the adults and developing unhatched embryos of a common aquatic snail (Lymnaea sps). The LC50 values of alpha-T for adults were found to be 39, 23 ppm and 77 ppb under dark, tube light and sunlight and for developing unhatched embryos the corresponding values were 620, 41 and 13 ppb. Erythrosin-B was found to be much less toxic under sunlight and dark, to both adults and embryos as compared to the toxicity of alpha-T. Potential use of such biodegradable and eco-friendly compounds of natural origin in mosquito control is discussed.