During contest competition, a competitor may persist in a given contest based on information regarding its own fighting ability (resource-holding potential, RHP), or that of its opponent. Although a number of models formalize the ways in which competitors are hypothesized to use RHP-related information to determine their persistence in contests, we focused on pure self-assessment and mutual assessment models in this study. According to pure self-assessment models, a competitor uses only information regarding its own RHP to determine its persistence in a contest. In contrast, according to mutual assessment models, persistence is based on information regarding a competitor's RHP relative to that of its opponent and therefore requires assessment between competitors. In this study, using size as a proxy for RHP, we tested whether the parasitoid wasp Nasonia vitripennis utilizes pure self-assessment or mutual assessment during pairwise, male–male contests. When we examined competitors of varied sizes, we found that the losing male's size was positively related to contest duration, but the winning male's size was uncorrelated with contest duration. When we examined contests in which competitors were size-matched, we found that the mean size of paired competitors was positively related to contest duration. These results suggest that male N. vitripennis engage in pure self-assessment during contests.