Copulation in E. cyathigerum can be divided into 3 stages. Stage I, which is correlated with the removal of rival sperm from the female’s storage organs, shows a greater degree of variability of duration than later stages. Such variability is dependent in part on the time of day: copulations occurring late in the day are significantly shorter. The number of rhythmic movements occurring during stage I is positively correlated with the duration of the stage whereas their frequency is negatively correlated. In stage II, during which the ejaculate is transferred to the female, the number of rhythmic movements remains more or less constant at about 20 and their frequency is negatively correlated with the overall duration of the stage. In stage III no movement occurs but the duration is positively correlated with that of stage I. Some of the factors which may account for the variability of copulation are considered.