Males of the damselfly Pyrrhosoma nymphula (Sulzer) defended perch sites from which they approached potential mates, tandem pairs and patrolling, unpaired males (interlopers). A simple residency asymmetry appeared to settle the majority of territorial disputes; resident males won 117 of the 120 recorded disputes (although they were smaller than interlopers on 56 occasions). On the three occasions when interlopers displaced resident males, the disputes were escalated, and resulted from confusion over residency. Short-term mating success was not significantly correlated with the observed number of territorial disputes in which a male was involved and size had no apparent effect on the observed mating success of males involved in territorial disputes. These findings are discussed in relation to game theory models for animal contests, and in the light of recent studies of size, territoriality and mating success in male dragonflies.