Sperm competition models suggest that ejaculate size (ES) is adjusted in relation to female fecundity and the risk of sperm competition, depending on the information males have about that risk. We tested these ideas in the migratory locust, Locusta migratoria, a species in which ES may be an important male fitness parameter. Copulation duration was positively correlated with ES. Males did not increase copulation duration but sperm transfer in the presence of competitors during mating. They did so only when they were reared in conditions that allowed female perception prior to mating. Males that prior to mating were kept with other males only did not show ES variation with regard to different sex ratios at mating. Increased female availability did not affect ES. A male size and condition related parameter was not significantly correlated to ES but older males delivered smaller ejaculates. Females of larger size were inseminated larger amounts of sperm. There was a positive correlation between female size and ES only for males of lower condition and lower relative testis weight but not for males of good condition or higher relative testis weight.