Mosquito larvae were examined to determine interspecific and interstrain differences in susceptibility to the larvicidal effects of the plant-derived phototoxin, alpha-terthienyl (alpha-T). The LC50 values were as follows: Aedes aegypti, 4 ppb; Ae. epactius, 6 ppb; anopheles stephensi, 14 ppb; malathion-susceptible Culex tarsalis (S), 12 ppb; malathion-resistant Cx. tarsalis (R), 16 ppb. Fluorescence studies indicated localization of alpha-T in the midgut epithelium and in the lumen of Malpighian tubules. Rates of elimination of tritiated alpha-T differed significantly between Ae. aegypti and Cx. tarsalis (S) larvae. Rate of 3H-alpha-T elimination was inversely correlated with susceptibility to the toxic effects of the compound. The toxicological significance of selective alpha-T accumulation and the importance of alpha-T elimination in determining sensitivity are discussed.