The correlation between malathion resistance and fecundity was investigated in the fruit fly, Drosophila melanogaster. Previous studies have shown that insecticide resistance is negatively correlated with reproductive components in insects. To investigate this relationship, the fecundity (egg production) was determined in 2 natural and 2 esterase 6 (Est-6) marker populations (4016 and 4211) and malathion resistance levels (LC50) were dei:ermined in the 4 populations of D. melanogaster. The resistance levels and Est-6 allozyme frequencies were compared to the results of 2 natural populations from Turkey (Antakya-Centre and Antalya-Serik) estimated in previous studies. In addition, malathion resistance levels (LC50) were determined in 2 mutant populations using the paper contact method and the results were tested with Finney's probit analysis. In the bioassays, 4016 marker stock was more susceptible than the Antakya population and 4211 was more susceptible than the Serik population. The daily mean egg production numbers of females were calculated and compared using variance analysis (P < 0.05). The most productive population was found to be Antakya, followed by 4016, Serik, and 4211 populations, in decreasing order. These results indicate that malathion resistance is positively correlated with fecundity of 2 natural and 2 Est-6 marker stocks of D. melanogaster.