Selection and hybridization experiments were conducted to explore the possibility of improving parasitization inTrichogramma. Several biparental stocks were hybridized and selected for their ability to live and reproduce within the temperature extremes likely to be encountered in the release area. In a uniparental stock, selection was made for locomotion, heat tolerance, a combination of locomotion and heat tolerance, and the ability to find hosts. Effects of these experiments were evaluated in field cages placed over cotton plants. No increase In parasitization resulted from the biparental crossings, and selection for temperature adaptation to the release area significantly reduced parasitization. The selection program for the uniparental stock produced positive results which were statistically significant at the 10% level. Greater significance might have been obtained if the program had been carried on for more generations. However, this type of selection program has limited application.