Members of the leiognathid subfamily Gazzinae, comprising approximately two-thirds of ponyfish species, are sexually dimorphic with regard to features of the light organ system (LOS). In Gazzinae, the circumesophageal light organ (LO) of males is enlarged and varies in shape compared with similarly sized conspecific females. In association with male species-specific translucent external patches on the head and flank, these sexually dimorphic LO features are hypothesized to be correlated with species-specific luminescence displays. Anatomical differences in LO shape, volume, and orientation, and its association with the gas bladder and other internal structures that function in light emission, are compared to observations of luminescence displays for every major lineage within Leiognathidae. We reconstruct the character evolution of both internal and external morphological features of the LOS to investigate the evolution of LO sexual dimorphism and morphology. Both internal and external sexual dimorphism in the ponyfish LOs were recovered as most likely to have evolved in the common ancestor of Leiognathidae, and likelihood-based correlation analyses indicate that the evolution of internal and external dimorphism in males is statistically correlated. Magnetic resonance imaging technology was applied to examine the unique internal LOs of ponyfishes in situ, which provides a new metric (LO index) for comparison of LO structure across lineages. J. Morphol. 2011. © 2011 Wiley-Liss, Inc.