The Indian urban malaria vector Anopheles stephensi Liston was selected for deltamethrin resistance for 25 generations (F₂₅) at larval and adult stages separately in the laboratory. There was roughly a 151-fold increase in the lethal concentration (LC)₅₀ and a 99-fold increase in the LC₉₀ in larval selection, when the F₂₅ was compared with the parent colony. Similarly, adult selection resulted in a 39-fold increase in the LC₅₀ and a 31-fold increase in the LC₉₀ in the adults. Knockdown bioassays conducted on adults (selected at the larval and adult stages) against the diagnostic concentration of insecticide-impregnated papers, namely, deltamethrin (0.05%), permethrin (0.75%), λ-cyhalothrin (0.05%), and cyfluthrin (0.15%), revealed that the adults selected at the adult stage were more resistant to deltamethrin and the other pyrethroids than those selected at the larval stage. A significant cross-resistance to DDT was noticed only in the adults selected at the adult stage, and no cross-resistance to malathion and propoxur was observed in the adults of both resistant colonies. Polymerase chain reaction studies revealed an occurrence of heterozygote level of kdr mutation (leucine to phenylalanine) in the adults selected at the adult stage. This event was not observed in the adults selected at the larval stage. Moreover, this is the first report on the occurrence of kdr mutation in Indian An. stephensi resistant to deltamethrin.