Pyrrhulina marilynae differs from all congeners (except Pyrrhulina zigzag) by presenting a conspicuous, dark primary stripe extending from tip of snout to middle caudal-fin rays, with a zigzag pattern along most body length (vs. primary stripe straight and narrow, mostly restricted to the head or not extending posterior to vertical through anal fin origin). Pyrrhulina marilynae differs from P. zigzag by the presence of a distinct, clear zigzag area lying between the primary stripe and the strongly pigmented dorsum; the lack of a hiatus on the primary stripe; and the absence of elongate dorsal-, anal- and caudal-fin rays in males (vs. clear zigzag area absent, and dorsum poorly pigmented; presence of a hiatus on the primary stripe at approximately fifth scale of fourth longitudinal series; rays of dorsal, anal and caudal-fin upper lobe distinctly elongate in males). The new species further differs from all congeners (except P. australis) by presenting nine principal rays on caudal-fin dorsal lobe, and 18 or less precaudal vertebrae (vs. 10 principal rays on caudal-fin dorsal lobe and 19 or more precaudal vertebrae). It can be further distinguished from P. australis by the absence of the posthcleithrum 2 (vs. postcleithrum 2 present). Pyrrhulina marilynae differs from all congeners (except Pyrrhulina zigzag) by presenting a conspicuous, dark primary stripe extending from tip of snout to middle caudal-fin rays, with a zigzag pattern along most body length (vs. primary stripe straight and narrow, mostly restricted to the head or not extending posterior to vertical through anal fin origin). Pyrrhulina marilynae differs from P. zigzag by the presence of a distinct, clear zigzag area lying between the primary stripe and the strongly pigmented dorsum; the lack of a hiatus on the primary stripe; and the absence of elongate dorsal-, anal- and caudal-fin rays in males (vs. clear zigzag area absent, and dorsum poorly pigmented; presence of a hiatus on the primary stripe at approximately fifth scale of fourth longitudinal series; rays of dorsal, anal and caudal-fin upper lobe distinctly elongate in males). The new species further differs from all congeners (except P. australis) by presenting nine principal rays on caudal-fin dorsal lobe, and 18 or less precaudal vertebrae (vs. 10 principal rays on caudal-fin dorsal lobe and 19 or more precaudal vertebrae). It can be further distinguished from P. australis by the absence of the posthcleithrum 2 (vs. postcleithrum 2 present) (Ref. 93568).