Heterosigma akashiwo is a raphidophyte known to produce blooms which can result in massive fish mortality. During the spring of 1998 a red tide prevailed in San Pedro Bay, within the Los Angeles and Long Beach Harbors complex, centered at the mouth of the Los Angeles River. Based on light microscopy of living cells, the predominant organism was isolated and tentatively identified as being H. akashiwo. Previously this alga had never been documented in San Pedro Bay. To provide confirmation that H. akashiwo had become established in San Pedro Bay, the isolate identity was determined by sequencing of the nuclear ribosomal internal transcribed spacer (ITS) region. Species determination was made by direct comparison to previously identified H. akashiwo. ITS sequence (isolate NIES 6). The result of this work was that there was complete sequence identity between the San Pedro Bay isolate and H. akashiwo. isolate NIES 6. This is the first confirmed documentation of H. akashiwo in Californian water, USA and an indication of continued expansion of H. akashiwo’s bloom range.