Variation in individual activity budgets may have important impacts for the long-term fitness of a population, yet our understanding of the factors shaping activity remains limited. Here, we report on intraspecific activity budget variation and the factors influencing it within a population of emerald basilisk (Basiliscus plumifrons) lizards in Costa Rica. Because behavioral variation may have a seasonal component, we monitor activity across both a wet and dry season. All basilisks exhibited similar frequencies of behavior throughout the day, with the exception of foraging rate. Adult females foraged more often than adult males or juveniles, possibly to satisfy higher expected energetic demands during the reproductive season. Juvenile and adult lizards occupied separate habitats characterized by significant differences in vegetation structure. In particular, juvenile lizards were more frequently observed in open, grassier habitats that were closer to water than adults. Juveniles may reduce their chances of predation by or competition with larger individuals in these areas, or may simply frequent those areas to take advantage of the size-dependent water-running ability characteristic of this species. As the area shifted from a wet to a dry period, juvenile activity significantly declined and by the end of the study very few juveniles were encountered. Adult lizards did not exhibit reduced activity per se, but instead shifted the timing of peak activity into early-morning and late-afternoon periods. Although activity budgets were largely similar among juveniles and adults, our findings highlight both ontogenetic variation in habitat use and the influence of seasonal variation on basilisk activity.