Laboratory selection pressure by parathion on Anopheles albimanus larvae from El Salvador resulted in 216X resistance to this chemical and in lower levels of cross resistance toward variously related organophosphates. Cross-resistance was influenced markedly by changes in either the alkyl or the aromatic moieties of parathion. Parathion and propoxur resistance in this strain, and possibly cross-resistance to closely related compounds, derives primarily from reduced sensitivity of acetylcholinesterase, as already demonstrated. Resistance toward dieldrin, which was present at significant levels at the time of strain colonization, has increased during selection by parathion.