The phenomenology of domoic acid (DA) production by strains of the Pseudo-nitzschia species complex has received considerable attention, leading to general observations that DA accumulation is stimulated by growth limiting or stress conditions. Although DA is a structural analogue of proline, no direct evidence is available linking DA and proline metabolism in Pseudo-nitzschia. In order to ascertain whether DA behaves as a functional analogue of proline, the free amino acid (FAA) composition of 5 species and 20 strains of Pseudo- nitzschia spp. from Monterey Bay were obtained by HPLC-UV profiling of their phenylthiocarbamyl amino acid (PTC-AA) derivatives. DA accumulation varied by 2-orders of magnitude among independent isolates of P. multiseries and P. australis, with the isolates of the latter species exhibiting consistently higher cellular yields of DA. Proline content was lower in cells accumulating high levels of DA (>1 fmole/cell) indicating a regulatory switch occurs between the biosynthetic pathways for these amino acids. All Pseudo-nitzschia species accumulated large pools of taurine (50% of total FAAs) when grown in Monterey Bay seawater (34 ppt). This osmolyte was not detected in other diatom species when grown under equivalent conditions. These trends indicate that the genus Pseudo-nitzschia may be characterized by a hypersensitive phenotype with respect to oceanic salinities. As taurine content covaried with DA accumulation in Pseudo-nitzschia, it may provide a useful biomarker for potentially toxic bloom events. Interactions between DA, proline and taurine metabolism will be discussed.