SYNOPSIS. Recent contributions to the theory of biogeography suggest that biogeographical hypotheses can potentially be derived from intraspecific patterns of morphological variation among populations. Morphological differentiation is demonstrated for meristic (serially repeated) and morphometric characters among populations of the euryhaline fish, Menidia beryllina, from the Mississippi Valley, an area for which the geological history is well known. Meristic and morphometric variation are associated with environmental variation, and meristic differences are noted between size classes within an isolated population. The results suggest that attempts to reconstruct historical area relationships will fail to the extent that geographical patterns of morphology are influenced by an exogenous component, when the direction of influence is unrelated to the true pattern of vicariance. Furthermore, temporal instability of characters will confound biogeographical hypotheses.