Significant genic differentiation occurred in samples of a stream population of Goodea atripinnis(Goodeidae) from a 1 km segment of the Rio Teuchitlan, Jalisco, Mexico. The lack of obvious selection gradients and physical barriers to migration over such short distances suggests that stochastic factors (seasonal reductions in stream flow, behavior) are involved in the differentiation. The extent of this phenomenon and its potential importance to the study of fish populations are discussed.