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SUMMARY Cavefish and their conspecific surface-dwelling ancestors (Astyanax mexicanus) are emerging as a model system to study the microevolution of development. Here we describe attributes that make this system highly promising for such studies. We review how the Astyanax system is being used to understand evolutionary forces underlying loss of eyes and pigmentation in cavefish. Pigment regression is probably explained by neutral mutations, whereas natural selection is a likely mechanism for loss of eyes. Finally, we discuss several research frontiers in which Astyanax is poised to make significant contributions in the future: evolution of constructive traits, the craniofacial skeleton, the central nervous system, and behavior.