Alexandrium minutum (strain AM89BM) has been grown in semi-continuous culture (0.2 vol. d super(-1)) in N-limiting (NO sub(3) super(-): PO sub(4) super(3-)=1.6 and 3.16), in P-limiting (N: P=160 and 80), and in N and P balanced (N: P=16) media. The toxin content in cells changed greatly according to the N: P ratio. Cells grown in N: P balanced condition showed an average total paralytic shellfish poisoning (PSP) content of 1.24 fmol cell super(-1). In N-limiting conditions, cells contained ca. 3 times less toxin with mean values of 0.41-0.45 fmol cell super(-1). In contrast, cells grown in P-limiting conditions contained on average 3.5 and 7 times more toxins than in the balanced N: P condition: 4.31 fmol cell super(-1) in the N: P=160 medium and 8.01 fmol cell super(-1) in the N: P=80 medium. The toxin content per carbon unit shows the same trend; the lowest content in N: P<16 conditions was 6.5 fmol PSP nmol C super(-1), whereas in the N: P=16 condition it was 3.3 times higher at 21.8 fmol PSP nmol C super(-1) and 9 times higher in N: P>16 conditions at 59.0 fmol PSP nmol C super(-1). Since present trends in nutrient loading are assumed to have skewed some historically stable situations towards higher N: P ratios, the stimulation of toxin production by P-limiting media has important ecological consequences; in niches where DIN: DIP is >16, populations of A. minutum sufficiently concentrated to represent a significant fraction of shellfish food could pose a serious problem