We have examined the relationship between Fe and blooms of the toxic dinoflagellate Alexandrium tamarense (Balech) (formerly Gonyaulax tamarensis var. excavata (Lebour)) using a chemical method that estimates the biological availability of Fe in seawater. The Fe requirement for optimal growth of A. tamarense in sequential batch culture (ca 3 nM 'available' Fe) was compared with Fe concentrations in waters of the Gulf of Maine, USA. Results indicated that Fe did not limit growth of the organism in nearshore coastal waters or over Georges Bank, but that the organism may have been Fe-limited in Gulf of Maine basin waters. The distribution of A. tamarense in the Gulf of Maine is consistent with these Fe data. Red tide outbreaks in the nearshore environment did not correlate with changes in total Fe or the estimated Fe availability. Although Fe did not appear to trigger outbreaks of A. tamarense in Maine coastal waters, the findings are consistent with suggestions that pulsed inputs of Fe may be important for the development of toxic dinoflagellate blooms in regions (e.g. Florida) where outbreaks are initiated offshore.