The cotton bollworm, Heliothis armigera Hubner, is the most serious pest of cotton in Thailand. A programme of screening chemicals for this insect in the field has been conducted since 1965 to discover the most effective insecticides and to establish a control strategy. This report gives a brief account of the history of insecticide use, beginning with endrin plus DDT, and toxaphene plus DDT during 1966-1983. Details of recent findings include some organophosphate compounds, one carbamate compound, one organic hydrocarbon, many synthetic pyrethroid compounds and some interesting insecticide tank mixtures. Insect resistance to the synthetic pyrethroids and some side-effects of certain insecticides are also discussed in the text. Alternate sprays between 2 different groups of chemical compounds is proposed for as one control measure.