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The migratory locust, Locusta migratoria L., shows phase polymorphism in which various morphological, physiological, and behavioral traits change according to population density. Information about the reproductive behavior, particularly sperm competition and male-female associations, has been obtained mainly from crowd-reared locusts. To understand the reproductive strategy in different phases, i.e., solitarious versus gregarious phase, we compared the P, (i.e., the proportion of offspring sired by the second male to mate), length of pre- and postcopulatory mounting and length of copulation between solitarious (isolated-reared) and gregarious (crowd-reared) adults of this locust. We observed that solitarious adults had a higher P, value, shorter precopulatory mounting, and longer copulation than gregarious adults. A significant positive correlation between length of precopulatory mounting and copulation was found in gregarious adults, but not in solitarious adults in the two geographic strains used. Little or no postcopulatory mounting was observed in either phase. Pairing of crowd-reared males and isolated-reared females indicated that males did not play a decisive role in determining these behaviors. The results demonstrate phase-related differences in reproductive strategy that are likely to be adaptive for locusts facing a large variation in chance of encounters between the two sexes.