Key to the species of Microsternarchus:

1. Distinct irregular suborbital blotches present; caudal filament length, 35.1–36.8% TL..................................................... Microsternarchus longicaudatus (lower Rio Amazonas near Juruti municipality, Pará state, Brazil)

1.a. Distinct irregular suborbital blotches absent; caudal filament length, 12.8–34.3% TL................................ 2

2. Small black spots overlying the mid-dorsal region of the body absent; caudal filament length, 29.3–34.3%TL; dentary terminates immediately after hook-like process on the posterodorsal portion................................................ Microsternarchus bilineatus (Río San José and Río San Bartolo, Río Guariquito, Guárico state, Río Orinoco basin, Venezuela).

2.a. Small black spots overlying the mid-dorsal region of the body present; caudal filament length, 12.8–20.9%TL; coronoid process of dentary continues posteriorly beyond dentary hook................................................................................................. Microsternarchus brevis (Middle Rio Negro basin, Amazonas state, Brazil).