In order to obtain more detailed information on the distribution of domoic acid-producing Nitzschia in Asian waters, Nitzschia-like diatoms were isolated primarily from the Philippines and established in culture for analysis by HPLC with fluorescence detection. Out of 58 isolates, 35 Nitzschia-like diatoms from the estuary areas of San Pedro Bay, Tacloban City and Manila Bay showed significant levels of domoic acid production (1.3-5.6pg/cell). These were identified as Nitzschia navis-varingica. Two isolates from the same locality did not produce domoic acid. Of the 21 isolates from Bulacan Estuary, Manila Bay, none produced domoic acid. They did, however, produce two substances that seemed to be domoic acid derivatives. One of the strains was mass cultured and the substances were extracted, purified and analyzed by LC-MS/MS, proton and (13)C NMR, and UV spectra. The produced substances were determined as isodomoic acids A and B. This is the first report of a diatom that produces isodomoic acids A and B as major toxin components.