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A high level of tolerance to malathion occurred in Aedes aegypti larvae of San Juan, Puerto Rico in 1978. The tolerance was 7-fold when compared with a susceptible strain. Malathion selection pressure in the laboratory on this tolerant strain resulted in resistance after only six filial generations. The resistance of the F6 generation was 10-fold when compared with the strain as it was in the field in 1969, and 19-fold when compared with a susceptible laboratory strain. The results, together with those of previous studies. suggest that resistance may be a major factor in situations in Puerto Rico in which malathion failed to control Ae. aegypti.