The ovariole development of laboratory-reared insecticide resistant and sensitive strains of the house fly, Musca domestica L. (Diptera: Muscidae) under different photoperiod regimes was examined. Flies were reared under laboratory conditions of 75 +/- 5% RH, 25 +/- 1 degrees C and 0:24, 6:18, 12:12, and 18:6 h L:D photoperiods. Each day, 20 female flies were collected from a cohort of females that had emerged on the same date. Ovarioles were dissected from the collected adults and microscopically examined to determine overall length and developmental stage. For the resistant strain, development was most rapid under the photoperiod regimes with more hours of darkness and steadily decreased with increasing hours of light. However, development of the susceptible strain was most rapid at both photoperiod extremes of 0:24 and 18:6 h L:D. Results also showed that the effect of both photoperiod and resistance status and the interaction between these factors had a significant effect on ovariole development.