Porthetria dispar(L.) males exhibiting pheromone orientation or searching behavior may be recognized by slow, slightly zigzag forward flight, with body held at an angle of roughly 45° to the horizontal plane and antennae directed up and forward. The mean search time for males searching alone at a natural pheromone source was 20.52±3.88 seconds; then males ended searching behavior. After termination of searching behavior, males held the body more horizontally, and forward flight was direct, rapid, and usually up or across the path of wind until they were lost from sight. Two males at a natural sex pheromone source touched wing tips in a mean time of 4.38±1.01 seconds of arrival of the second male. Searching behavior was terminated by rapid wing touching within two seconds in one or both males in 86 percent of the contacts.The evolutionary significance of this aggressive competition may be that it tends to disperse males from dense to sparse populations, thereby increasing the chance of males mating with isolated females, which have greater reproductive potential than females from dense populations.