Resistance levels to deltamethrin were measured in 5 natural populations of Anopheles sinensis. The median lethal concentrations (LC50S) of deltamethrin in these populations were higher than those in susceptible strains originating from the same populations, especially in the Wenzhou population, which had a resistance ratio (RR50) of 11 relative to its susceptible strain. Resistant strains were selected with deltamethrin for 12 generations. Resistance levels in resistant strains were 130 to 190-fold higher than in susceptible strains, and 10 to 40-fold higher than in natural populations. Response of selection (R) in the resistant strain from the Wenzhou population was less than 0.1, and those in resistant strains from other natural populations were more than 0.1. This suggests that a resistant strain from a natural population with higher resistance has a lower increase in RR than a resistant strain from a natural population with low resistance under identical insecticide selection. These results are discussed in relation to mosquito control strategies.