The live weight of adult Anopheles culicifacies was affected by variation in rearing conditions and using either unfed or blood fed females for insecticide tests. Mosquitoes homozygous resistant to dieldrin were the main genotype used on 4% dieldrin treated papers for exposure times from 5 to 16 hours. LT50 determinations showed significant heterogeneity but clearly heavier blood fed adults showed less susceptibility to dieldrin. All the laboratory reared adult sizes could be found in the field as shown by collections from one village during 1976. The implication of such weight and trophic status variation is discussed with respect to discriminating dosages and the degree of resistance involved.