Domoic acid (DA), a neurotoxic amino acid, was detected by HPLC-fluorescence analysis in 5 monoclonal cultures of Nitzschia sp. nov. (Bacillariophyceae), isolated from a shrimp culture-pond at Do Son, Vietnam in 1997. DA levels of the cultures ranged from 1.2 to 3.1 (average 2.3) pg/cell, which was within the amounts reported for Pseudo-nitzschia multiseries. Confirmation of DA was performed by ESI MS after purification of the DA-like substance which was extracted from the cells of 3 L culture of one strain. DA production of the new species were investigated by batch culture experiments under axenic and non-axenic conditions. Like in many Pseudo-nitzschia species, DA production of both cultures started from late exponential growth phase and reached a maximum at early stationary growth phase followed by gradual decrease. In contrast to P. multiseries, no significant difference was observed in DA production between axenic and non-axenic cultures. Five monoclonal cultures of the same species were also established from Ha Long Bay, Vietnam in 1998. DA production of them were also confirmed by HPLC analysis.