Mesopelagic (open ocean, 200-1000 m daytime depth) fishes are important consumers of zooplankton and are prey of oceanic predators. Two dominant mesopelagic fish families, Myctophidae and Sternoptychidae, occupy a similar daytime depth, but different nighttime depths. Myctophids undertake diel vertical migrations, while the sternoptychid genus Sternoptyx does not. The relationship between parasites and gut contents provides insights into ecological processes occurring within assemblages, as prey items are often vectors for parasites. This study examined the differences between the prey contents and parasite fauna of these two contrasting families in the Gulf of Mexico. Results showed that the non-migrating Sternoptyx fed upon a broader range of taxa than myctophids and appeared to transition from a zooplanktivorous to a micronektonivorous feeding style with increasing size. No feeding guild shift was observed in the vertically migrating myctophids, but their ration increased with body size. In both families, parasites were more abundant among higher size classes. The more diverse feeding predator, Sternoptyx sp., harbored a lesser diversity of parasites than the more selective feeding predator. These data can be used to develop and refine models aimed at understanding ecosystem structure and resilience to large-scale disturbances such as the Deepwater Horizon oil spill.