Mussel shell mounds surround all offshore oil and gas platforms in California. These biotic reefs are formed when large clumps of mussels are dislodged from the superstructure. In 1997, we surveyed the fish assemblages on the mussel mounds surrounding seven platforms in the Santa Barbara Channel and in the Santa Maria Basin, California. The objectives of this study were (1) to document the fish assemblages on the mussel reefs, (2) to investigate the spatial patterns of use of parts of mussel reefs by various fish species and (3) to compare species assemblages, population densities and fish sizes on the mussel reefs with those on adjacent platform bottoms. We observed at least 35 species on the mussel mounds, 18 of which were rockfishes (genus Sebastes). Most of the species that were found both in large numbers and were encountered at a number of mussel mounds were solitary, benthic forms. Most species appeared to be non-randomly distributed among parts of the mussel mounds with different percent shell cover. All species combined and all rockfish species tended to be slightly but significantly over-represented on areas of 80−100% cover (all species: χ 2 = 227, n = 5, P χ 2 = 211, n = 5, P −2) and mean lengths of fishes were all less on the mussel mounds than on the platform bottoms. However, cluster analysis revealed that the species composition on each mussel mound is more similar to its adjacent platform bottom than to other mounds. There did not appear to be a distinct "mussel mound community", instead the mussel mounds should be considered as an integral part of the oil platform system.