Key to the species of Gymnocorymbus

1 Thirteen to sixteen gill rakers on lower limb of first branchial arch.............................................. 2

- Eleven (rarely twelve) gill rakers on lower limb of first branchial arch....................... G. bondi (Orinoco basin)

2 Pelvic-fin rays i,6,i or i,7............... G. thayeri [Amazon basin (except Rio Madeira) and rivers of northeastern Brazil]

- Pelvic-fin rays i,5,i or i,6.............................................................................. 3

3 Premaxillary and dentary teeth tricuspidate, five teeth in inner row of premaxillar.............. G. ternetzi (Paraguay basin)

- Premaxillary and dentary teeth pentacuspidate, six to seven teeth in inner row of premaxillar.... G. flaviolimai (Rio Madeira)