Domoic acid (DA), a potent neurotoxin, is synthesized by certain members of the ubiquitous marine diatom genus, Pseudo-nitzschia. We have recently detected elevated concentrations of DA in phytoplankton field samples from the northern Gulf of Mexico. In searching for a possible source of the toxin, we have detected DA in cultures of Pseudo-nitzschia pseudodelicatissima isolated from this region and confirmed its presence using tandem mass spectrometry, preceded by liquid chromatographic separation of the toxin (LC-MS/MS). Unlike other toxic Pseudo-nitzschia species examined previously (e.g., P. multiseries, P. australis), cellular levels and production of DA in these P. pseudodelicatissima strains were highest in early exponential phase, while population growth rate was maximal. The maximum cellular DA activity measured in the culture by receptor binding assay was 34 fg DA equiv. cell-1. No net accumulation of toxin was evident in stationary phase, yet the distribution of DA shifted from predominantly intracellular to mostly extracellular during this stage of growth. This study unequivocally establishes P. pseudodelicatissima as a source of DA in the northern Gulf of Mexico. Moreover, our work suggests that rapidly growing, rather than nutritionally stressed, populations of this and possibly other Pseudo-nitzschia spp. may contain maximum cellular DA quotas, which is in contrast to the current paradigm of Pseudo-nitzschia toxin dynamics.