The skull of the characid fish Astyanax mexicanus is described based on twenty alizarin-stained adult specimens from Río Salado, Río Conchos, Río Alamo, and Río San Juan populations, all of them Rio Grande tributaries in Northeastern Mexico. The skull has a circular shape in lateral view. The second infraorbital is triangular and never overlaps the inferior margin of the third infraorbital. The third infraorbital never reaches the laterosensory canal of the preopercular bone. The supraoccipital is short. The palatines, ectopterygoids and mesopterygoids lack teeth. We describe two features not reported in Astyanax before: teeth on the second suspensory pharyngeal and posterior gill rakers on the four gill arches. Differences in skull osteology between A. mexicanus and other described species of Astyanax are denoted. Inglés

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