Vertebral numbers of the emerald shiner Notropis atherinoides collected from approximately 100-mile intervals of the Ohio River during 1957-1959 were analyzed using x-rays. Counts ranged from 36 to 42 with an overall mean of 39.58 vertebrae. There was a consistent gradual cline in numbers of vertebrae with significant differences in mean numbers. Numbers were smallest near the source and largest near the mouth of the river. This is a reversal of the usual trend of greater numbers of vertebrae at higher latitudes, but it may be consistent with Jordan's Rule in that habitats in the upper (more northerly) reaches of the river may warm more quickly, and embryonic development may proceed there at higher temperatures than in the lower reaches.