Monitoring data from estuaries can provide us with important insights into ecosystem functioning and community trends that might otherwise go undetected. The San Francisco Bay National Estuarine Research Reserve (NERR) is one of a few reserves piloting a long-term biomonitoring project to assess estuarine crab communities across the U.S. Following the protocol developed at the Elkhorn Slough NERR, we are sampling quarterly across the estuarine gradient in China Camp State Park and Rush Ranch, two reserves that cover a range of San Francisco Estuary's salinity gradient. We are interested in detecting novel invaders and changing abiotic conditions that may affect estuarine communities. Preliminary data from China Camp indicate that the invasive European green crab, Carcinus maenas (Linnaeus, 1758) represented 28-50% of the catch per transect by biomass and 5-12% by number of individuals. The native mud crab Hemigrapsus oregonensis (Dana, 1851) represented the vast majority of the remaining catch. By monitoring abundance, sex, and size of the native and invasive crabs over time, and coupling these data with the NERR water-quality monitoring program, we will not only be provided with an “early warning” indicator of new invasions, but will test hypothesis about invasive species and their relationship to environmental and biotic parameters. Over time, the results will allow us to observe abundance trends and changes in other characteristics such as intra-site distribution and aggregation patterns. As this monitoring program is adopted system-wide across the NERRs, it will enable geographic comparisons of the dynamics of these invasions.