Field experiments were conducted during 1997 and 1998 cotton seasons to study the effect of individual synthetic pyrethroids sprayed three times at 15 days interval on the population density of the pink bollworm larvae infesting cotton bolls. Data shown that ?.-cyhalothrin and cyhalothrin proved to be the most effective in reducing pink boll­worm incidence on cotton among the tested compounds. They caused 79.29 and 77.45% reduction of pink bollworm larval population in 1997 and 74_97% and 71.97% in 1998, respectively. On the other hand, del­tamelhrin ad cyfluthrin were the least effective insecticides, giving 59.44 and 54.95% in 1997 and 50.00 and 46.94% reduction in 1998, respectively. Also, the response of the field strain larvae to the same insecti­cides at the end of the rotational chemical control programme was eval­uated in laboratory. Data indicate that the larvae of 1998 showed more tolerance compared to that of 1997 season. The tolerance ratio ranged between 1.01 and 1.32 at LC₅₀ level, where it ranged between 1.08 and 1.69 at LC₅₀.