Black abalone (Haliotis cracherodii) have experienced mass mortalities along the coast of California and are now protected under the US Endangered Species Act. As part of a long term monitoring program called MARINe (Multi-Agency Rocky Intertidal Network), we have documented their decline due to poaching and the disease, “withering syndrome”. Since 1992 we have monitored abalone abundance and size structure in permanent plots at 27 sites along the central coast. Recent surveys of 326 areas at 54 sites allowed us to estimate the population size and determine the amount of suitable black abalone habitat along the California mainland. The last extant large and healthy populations exist in the Monterey Bay National Marine Sanctuary. In diseased areas we find almost no recruitment and the average density of mature abalone is below the level for reproductive success. Our data on recruitment and habitat alteration post-disease suggest that their recovery in such areas is likely to be very slow without intervention. Recent studies also imply that the surviving individuals in southern California may be disproportionately susceptible to sea otter predation, should otters reoccupy that area. Our data have been used by the National Marine Fisheries Service to designate critical habitat and to aid the recovery of black abalone. Population and community consequences of the loss of this important species could not be assessed without a spatially expansive long-term monitoring program.