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Two laboratory populations of Drosophila melanogaster were exposed to the insecticides ethyl-parathion and fenthion respectively. Three parameters were studied in each generation: resistance to the insecticide (LC 50), AChE activity, and allele frequencies at the Est-6 locus. ln both treated populations a significant increase in the resistance was observed within a few generations. This rise in the resistance levels was accompanied by parallel changes in the two esterases AChE and Est-6. With AChE the enzyme activity became considerably higher indicating either structural modification of the enzyme molecule or mutations in the regulatory system of the AChE gene locus. At the Est-6 locus the S allele was eliminated during the selection process which might be due to greater sensitivity of the Est-6s allozyme.

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