Morphology is one of the most common methods used to identify among congeneric species, but its use can be problematic when spatial patterns of phenotypic variability are unknown. We examine spatial variability in the morphology of two sympatric species of algae (Fucus vesiculosus and F. spiralis) on large (>100 km) and small (<10 km) scales and determine the extent to which they can be identified based on morphology. These species could generally be distinguished based on gross morphology on large spatial scales, but on small scales some individuals exhibited much overlap in the gross morphology. This finding is not surprising given that hybridization and introgression in these species is common. Although there were some consistent patterns in morphology between species, many were species-specific. Similarly, there were few common spatial patterns when individual measures of morphology were analyzed, suggesting that selective pressures may act on each species independently. These results have implications for the use of morphology in identifying congeneric species. Given the existence of individuals of intermediate morphology, it is likely that species are often misidentified when spatial variability in morphological distinctness is not considered. In particular, stipe width may prove to be a valuable predictor of species identification for non-reproductive and young individuals, as it was the only morphological variable to vary consistently between the species.