Key to the species of Isorineloricaria

1a. Caudal fin depth to head length ratio 16.5–19.1%................................................ I. spinosissima

1b. Caudal fin depth to head length ratio 20.6–30.1%........................................................... 2

2a. Sharp ridges on compound pterotics................................................................. I. villarsi

2b. Rounded ridges on compound pterotics................................................................... 3

3a. Caudal peduncle width to pectoral spine length ratio 31.5–56.2% (separates 95% of individuals examined)..... I. tenuicauda

3b. Caudal peduncle width to pectoral spine length ratio 59.6–65.2% (separates 95% of individuals examined)....... I. acuarius