Risk of sperm competition is becoming increasingly appreciated as a determinant of male reproductive behavior in evolutionary ecology. That is, a male is under selection to adjust his ejaculate investments into a female depending on the mating histories of both. I investigated such behavior in the Australian Mallee dragon lizard (Ctenophorus fordi). Females in this species mate repeatedly and apparently indiscriminately with several partners in succession. Intuitively, a male mating subsequent to a rival should mate longer if doing so transfers more competing spermatozoa or seminal fluids that may act as paternity guards. I tested the prediction that males mate longer with females known to have mated recently with other males in a simple experiment using wild-caught lizards kept in outdoor enclosures. Males allowed to observe a female's first copulation with rival males and then copulate with the same female remained in copula 60% longer (15.9 s on average) than the males in the first matings (9.9 s on average). In the reciprocal trials in which males in females' second copulations could not observe the first male's copulation, second males did not mate for longer than first males. Furthermore, copula duration was positively correlated with transferred ejaculate volume. Thus, the results suggest that males may prolong copulation to become more competitive when under increased risk of sperm competition.