Adults of the parasite Apanteles sp. group ultor were held under constant temperatures (15, 20, 26, 30 degree C) each within plus or minus 1 degree C. Total progeny production was significantly greater at 26 degree C, averaging 85 per female and decreased at 15 degree C and 30 degree C. Mating behavior showed that both sexes were a polygamous. The range of constant temperature did not greatly alter the progeny sex ratio when female parent had been mated. At 15 degree C, male activity was adversely affected when both sexes were exposed to it without previous mating, resulting in a higher number of male progeny. A temperature of 26 degree C was the most appropriate for normal sex ratio.