Yessotoxin (YTX) is a newly discovered type of phycotoxin that is commonly produced by two dinoflagellates, Protoceratium reticulatum and Lingulodinium polyedrum. P. reticulatum has been confirmed to produce YTX and other analogues from isolates in New Zealand, Norway, Spain, Italy, Canada, the United Kingdom and the United States. L. polyedrum has been determined to produce YTX in isolates from Italy, the United Kingdom, Ireland and, most recently, Spain. L. polyedrum is present most years off Baja, California, and bloom events of this species in southern California coastal waters have been recorded as far back as 1901. Three cultures of L. polyedrum isolated from southern California coastal waters were tested for the presence of YTX using Biosense Laboratory ELISA kits. Yessotoxin-like activity was detected in the particulate phase of two out of three cultures. Toxin was also detected in the dissolved phase. However, this is probably the result of salt matrix effects rather than measurable toxin. This is the first study to confirm the potential for YTX production in California isolates of L. polyedrum.