Diagnosis: Crenicichla ploegi is placed in the C. saxatilis species group by having a humeral blotch even in the smallest specimens observed, less than 80 scales in the E1 row (58–71), dimorphic males with many light dots scattered on the flank, and infraorbital series with very slender lacrimal bone and 5 separate post-lacrimal tubular infraorbitals; distinguished from all other species of the C. saxatilis group by the presence of dark spots and vermiculations on snout, interorbital area and dorsally on head in many adults up to 146.6 mm SL vs. dark markings absent or present only in juveniles but absent in adults (head uniformly dark gray in adults from 80 mm SL), and by the presence of a deep and strongly pigmented lateral band present in both juveniles and adults, occupying a depth of 4–6 horizontal scale rows vs. lateral band narrower, occupying a depth of 2–3 horizontal scale rows, and conspicuous only in juveniles and smaller specimens, faded or absent in larger specimens; Crenicichla ploegi is more elongate (body depth 17.8–21.7% SL) and has more scales in the E1 row (58–71 scales) than C. brasiliensis, C. britskii, C. frenata, C. isbrueckeri, C. labrina, C. lepidota, C.proteus, C. santosi, C. saxatilis and C. semicincta (body depth more than 21% SL, and fewer than 56 scales in the E1 row); Crenicichla ploegi differs from C. alta, C. albopunctata, C. anthurus, C. coppenamensis, C. pydanielae and C. sipaliwini by having the humeral blotch situated almost entirely below the anterior branch of the lateral line vs. humeral blotch with center on the level of the anterior branch of the lateral line (Ref. 119086).