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The effect of parasitoid age and two instars of the host Helicoverpa zea (Boddie) were investigated for the endoparasitoid, Microplitis croceipes (Cresson). Third and fourth instars of H. zea were exposed to three different age ranges (3 to 5, 6 to 8, and 12 to 15-d-old) of mated adult female M. croceipes. No significant differences were shown in rate of parasitism regardless of parasitoid age or host instar. Significant differences (P < 0.05) were found between parental age and adult wasp emergence, sex ratio, and number in pupal stage entering diapause. Six to 8-day-old parasitoids parasitizing third instar hosts yielded the highest percentage of adult emergence (47.5 ± 14%; X̄ ± SD), the highest percentage of females (79%), and the lowest percentage (4.5 ± 6%; X̄ ± SD) entering diapause during the test.