Laboratory and field cage studies were conducted to determine the degree of successful parasitization by selectedTrichogrammastocks for the eggs ofHeliothis zea(Boddie), the bollworm, andTrichoplusia ni(Hübner), the cabbage looper.The laboratory studies showed significant increases in female longevity, number of eggs successfully parasitized, and progeny production whenH. zeaeggs were used as hosts.Two types of field cage experiments were conducted in cotton fields. The 1st type, which had eitherH. zeaorT. niegg cards in a given field cage, indicated that twice as manyH. zeaegg cards were found. The 2nd type of experiment had both hosts in varying proportions in each cage, and showed that a higher proportion ofH. zeaeggs were successfully parasitized even whenT. nieggs were more abundant.