The effects of coastal acidification on the growth and toxicity of the saxitoxin-producing dinoflagellate Alexandrium fundyense were examined in culture and ecosystem studies. In culture experiments, Alexandrium strains isolated from Northport Bay NY, USA, and the Bay of Fundy, Canada, grew significantly faster (16 -190%; p<0.05) when exposed to elevated levels of pCO2 (~ 800- 1900?atm) compared to lower levels (~390?atm). Exposure to higher levels of pCO2 also resulted in significant increases (71 - 81%) in total cellular toxicity (fg STX eq. cell-1) in the Northport Bay strain, while no changes in toxicity were detected in the Bay of Fundy strain. The positive relationship between pCO2 enhancement and elevated growth was reproducible using natural populations from Northport; Alexandrium densities were significantly and consistently enhanced when natural populations were incubated at 1500 ?atm pCO2, a value at the upper range of those recorded in Northport Bay, 390 - 1500 µatm. During natural Alexandrium blooms in Northport Bay, pCO2 concentrations increased over the course of a bloom to more than 1700?atm and were highest in regions with the greatest Alexandrium abundances, suggesting Alexandrium may be further exacerbating acidification or be especially adapted to these extreme, acidified conditions. The co-occurrence of Alexandrium blooms and elevated pCO2 represents a previously unrecognized, compounding environmental threat to coastal ecosystems. The ability of elevated pCO2 to enhance the growth and toxicity of Alexandrium indicates that acidification promoted by eutrophication or climate change can intensify these, and perhaps other, harmful algal blooms.