Morphometric analyses of Agosia chrysogaster (Girard) indicated a northern morph native to Bill Williams, Gila, Sonoyta and de la Concepcion basins of Arizona, New Mexico and Sonora, and a southern form from Willcox Playa of Arizona and Rios Sonora, Yaqui, Mayo, Fuerte and Sinaloa of Sonora and Sinaloa, Mexico. The latter is smaller, and less sexually dimorphic, but has longer pre- and postdorsal body lengths. Populations in the geographically intermediate Rios Sonoyta and Sonora are

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... ally intermediate. Males differ more between morphs than do females. Meristic characters show considerable overlap between morphs, but the northern form has higher mean lateral line scale counts. Highly tuberculate nuptial males, characteristic of the northern morph, were not found in the south, nor were "spawning" pits characteristic of breeding activities of the former. Morphs differ on a multivariate axis on which temporal variation at single localities is also reflected. Distances among some intra-locality samples on this axis were greater than least inter-morph morphological distances. Measures of morphological dissimilarity were weakly correlated with inter-sample differences in elevation, latitude, and longitude, but more highly correlated with an index of hydrologic isolation among localities. Differentiation among basins thus appears to reflect hydrographic isolation, rather than ecological conditions. Electrophoretic data on A. chrysogaster produced relationships patterns largely incongruent with results of the morphological analyses, and with unexpected geographic area relationships.