The nature of chemical defenses in poison frogs has been explored in a variety of species, and most studies focus on the types of chemical defenses and their sources. The defensive compounds of frogs are stored in dermal granular glands that have been described for several species that are chemically protected from predators and/or microorganisms. Gland ultrastructure is known for some species of dendrobatoid frogs, but the relationship between body size and chemical defense has heretofore not been explored. It might be expected that the capacity for defensive protection increases as a function of body size, especially given the fact that juvenile poison frogs are known to have smaller quantities of alkaloids than adults. We examined poison glands histologically in a sample of the poison frog Oophaga pumilio to determine if the physical basis of the defensive system changes as a function of body size. We measured average gland size, estimated the number of glands, and calculated the density and percentage of skin area occupied by glands in a patch of dorsal skin for 25 individuals. For males and females, the size, number and percentage of skin area occupied by poison glands increased allometrically as a function of body size, whereas poison gland density decreased with body size. Adults have a larger capacity to store alkaloids and more of their dorsal skin is associated with poison glands as compared with juveniles, which may translate into greater protection from predators in adults and could explain why adults are more apparent (active above the leaf litter) than juveniles at our study site in north-eastern Costa Rica. Furthermore, juveniles and subadults may benefit from automimicry because they resemble adults in appearance.