During copulation in the alfalfa butterfly, Colias eurytheme, a male fills two sacs in the female's reproductive tract. The appendix bursae is filled first with a white milky fluid and then the corpus bursae is filled with a spermatophore that contains the sperm and other materials. The sperm are deposited last in the female's reproductive tract. Recently-mated males produce less material during copulation and copulate for much longer times. Large males pass more material and old males copulate for longer times. Variation in the duration of copulations observed in the field suggests that males mate about once every five days. A release-recapture experiment with virgin females indicates a similar mating frequency for that sex. The discussion relates these data to the known patterns of mate choice and reproductive behaviour in butterflies and moths.