Diarrhetic Shellfish Poisoning Toxins (DSTs) are produced by the marine dinoflagellate, Dinophysis. DSTs can bioaccumulate in shellfish and cause gastrointestinal illness when humans consume high levels of this toxinches Although not regulated in the U.S., recent studies in Washington, Texas, and New York suggest DSTs may be widespread throughout U.S. coastal waters. This study describes a four-year time series (2013-2016) of Dinophysis concentration and DST level in California mussels (Mytilus californianus) from Santa Cruz Municipal Wharf (SCMW) in Monterey Bay, California. Results show a maximum Dinophysis concentration of 9,404 cells/L during this study and suggest Dinophysis persists as a member of the background phytoplankton community throughout the year. DSTs in California mussels were found in persistent low levels throughout the course of this study, and exceeded the FDA action level of 160 ng/g 19 out of 192 weeks sampled. Dinophysis concentrations alone are a positive but weak predictor of DST level in California mussels, and basic environmental variables (temperature, salinity, and nutrients) do not sufficiently explain variation in Dinophysis concentration at SCMW. Overall, this study demonstrates that Dinophysis concentrations on the central coast of California, at SCMW, are producing DSTs that accumulate in local shellfish throughout the year, occasionally reaching levels of concern.