Signaled presentations of a rival male produce an aggressive conditional response in several species of fish. Although conditioning of such species-specific display behavior has been described previously, the biological function of this learning phenomenon has remained unknown. I present experimental evidence that signaled territorial intrusion enables a male blue gourami to defend his territory more aggressively than when the intruder is unsignaled. In Experiment 1, pairs of territorial males, with different conditioning histories, confronted one another for the first time. One member of the pair previously had received Pavlovian conditioning, whereas the other pair member had received explicitly unpaired presentations of the same signal--conditional stimulus (CS)--and rival male--unconditional stimulus (US). In the subsequent encounter, which was signaled by CS presentation, Pavlovian males delivered significantly more bites and tailbeatings than did their control group opponents. Experiment 2 relied upon a different control procedure, a US-only condition, but, again, Pavlovian males enjoyed a significant aggressive advantage. These results suggest an important ecological role for Pavlovian conditioning.