ABSTRACT: Previous studies have reported dramatic declines of midwater fishes in the southern California Current System (CCS) in response to reduced deepwater oxygen concentration, and many CCS endemics and cool-water affinity taxa have also declined since 1969. Here we show that these abundance changes are associated with changes in the diversity of the CCS fish community. Species richness was positively associated with periods of relatively high midwater oxygen concentration and with the warm phase of the Pacific Decadal Oscillation, the multivariate ENSO index, and warmer near-surface temperature. While the decline of CCS endemics and cool-water affinity taxa was not associated with a change in species richness, it significantly enhanced the evenness of the fish community by markedly reducing the abundance of dominant taxa. Community structure overall, however, changed relatively little: the rank order of the 15 most abundant taxa changed little between periods of high and low species richness and evenness. Overall, fish community structure in the CCS thus appears to have remained stable in recent decades, despite marked changes in abundance of entire suites of species and significant changes in several biological diversity indices. The increase in the Shannon-Wiener index of evenness associated with the marked decline of several dominant taxa in the CCS indicates that changes in simple diversity indices need to be interpreted cautiously: increased diversity cannot be equated with improved ecosystem functioning.