Offshore petroleum platforms in California's Santa Barbara Channel have been shown to be of ecological importance for local fish populations; however, before our investigation, no data existed on the fish assemblages associated with the platforms on the San Pedro Shelf further south. Our surveys of these platforms in 2006 and 2007 indicate that there is a core group of warm–temperate nearshore reef species in the midwater depth (<30 m), including California sheephead Semicossyphus pulcher, blacksmith Chromis punctipinnis, garibaldi Hypsypops rubicundus, opaleye Girella nigricans, and kelp bass Paralabrax clathratus. Platforms located further from shore (i.e., offshore platforms [>14 km]) had a strong seasonal presence of pelagic fishes (primarily jack mackerel Trachurus symmetricus) and recruit blacksmiths. At platforms closer to shore (i.e., inshore platforms [<4.8 km]), sea basses (Serranidae) and surfperches (Embiotocidae) were predominant; only slight seasonal variations were observed, which were attributed to prespawning aggregations of barred sand bass Paralabrax nebulifer. At the offshore platforms total fish density had a negative relationship with depth but a positive relationship with water temperature; conversely, total fish biomass exhibited a positive relationship with depth but a negative relationship with temperature. In contrast, at the inshore platforms both total fish density and biomass increased with increasing water temperature. With the exception of blacksmiths, nearshore reef fishes showed significantly greater densities of adults than juveniles. Furthermore, some species were observed spawning at these structures. Therefore, the San Pedro Shelf platforms may provide suitable habitat for several species of nearshore reef fish. Removing the upper levels and superstructure of the offshore platforms to a depth of 25 m (the partial removal decommissioning option) would eliminate critical habitat for several fish species and result in a potential loss of 95% of the total fish density and 77% of the total fish biomass, thus reducing the productivity advantages of some of these structures.