Phenotypic traits often differ among conspecific populations inhabiting different environments. These morphological types may represent unique genotypes that are best suited to local conditions, or they may result from individual responses to local environmental conditions and thus represent plastic phenotypes. The barnacle Chthamalus fissus, commonly found in the upper intertidal of southern California and Baja California, Mexico, exhibits three morphs: one with an oval operculum, one with a narrow, slit-like operculum, and a relatively uncommon bent form with the operculum opening on one side. The aim of this study was to determine if differences in shell morphology among two populations of C. fissus separated by approximately 100 km result from local adaptation or are the result of plasticity in development of the barnacle shell. A reciprocal transplant experiment was conducted in which rocks with attached juvenile barnacles were transferred among populations from La Jolla, California, USA, where adults exhibit an oval operculum, and Ensenada, Baja California, Mexico, where adults exhibit either a narrow operculum or the bent morphology. The results indicate that barnacle morphology is determined by local conditions and is not due to genetic differences between the two populations. Results of a field experiment examining predation by the snail Mexacanthina lugubris lugubris suggest that oval morphs are more vulnerable than narrow morphs to this predator. Environmental factors that may be responsible for inducing this morphological variation are discussed.