Adult male animals are commonly believed to exhibit higher activity than other conspecifics, but little information exists to compare their activity with that of other conspecifics of similar size. Here we compare the activity of adult male and similar-sized juvenile female crab spiders Misumena vatia (Araneae, Thomisidae). Adult males moved farther and more frequently than juvenile females of similar size (fourth instar) that were not affected by impending molt. Juvenile females influenced by impending or recent molt did not move as far or as frequently as nonmolting juveniles, even though their exoskeletons were hard enough to permit rapid movement. A small sample of penultimate males, of similar size to the adult males and juvenile females, exhibited activity patterns similar to the juvenile females. All of these data indicate that the high activity level of adult males is not a simple manifestation of behavior that is solely a function of size. We suggest that the high activity levels of the adult males facilitate search for scarce, cryptic mates.