Domoic acid (DA), the responsible agent for Amnesic Shellfish Poisoning, contains several carboxylic acid residues that could potentially bind trace metals such as iron. To investigate if DA production was affected by the iron status of the cell, replicate cultures of the diatom Pseudo-nitzschia multiseries were grown under trace-metal clean conditions containing 0, 0.12 µM and 11.7 µM added iron. Cell growth was monitored by in vivo fluorescence, extracted chlorophyll, and visual cell counts; DA production was measured by the FMOC HPLC method. All three iron treatments showed similar initial growth rates and resulted in stationary phase densities of greater than 10⁵ cells/mL after 10 days in culture. In contrast, DCMU-enhanced fluorescence indicated that the cultures without added iron were iron-limited by day 18, as evidenced by an Fv/Fm ratio of 0.2 as compared to the iron-replete cultures (11.7 µM Fe) with an average Fv/Fm ratio of 0.5 throughout the culture period. Equally dramatic changes were seen in the total chlorophyll per cell and the mean DA production per cell. Cultures without added iron showed a marked drop in the cellular chlorophyll a content in stationary phase and never produced more than 5 pg DA/cell (<1000 ng/mL). In contrast, iron-replete cells contained 5-10 times more chlorophyll per cell, and DA production increased steadily throughout stationary phase to nearly 50 pg/cell (5500 ng/mL). The intermediate level of added iron (0.12 µM) showed intermediate results: the Fv/Fm ratio and chlorophyll content varied considerably throughout stationary phase and DA reached 20-25 pg/cell (~3500 ng/mL). These results indicate that the lack of available iron strongly inhibits the ability of P. multiseries to produce DA. This decrease in production suggests that DA is not produced as a chelator to increase the availability of iron to the cells. The cause of this decrease is unknown, but the biosynthesis of DA requires both nitrogen and energy, two resources that are likely to be limiting under iron-deficient conditions.