Last-male sperm precedence is known in many arthropods. We set out to investigate whether it occurred in the aphid Myzus persicae, by mating females successively with two individual virgin males of different clones. In each case, the female used was of a clone different from either male clone. Three clones were used, 031, 047 and 066, selected so that a single microsatellite locus could discriminate the paternity of each embryo. In every case, males of 031 fathered more eggs than males of 066 or 047, and males of 066 always fathered more eggs than 047, regardless of mating order. Hence there exists a fitness difference among the males. We have investigated parameters of size and mating behaviour in an endeavour to determine the physiological or behavioural basis of the fitness differential.