Alexandrium catenella is a PSP producing dinoflagellate which blooms in organic-enriched (from terrigenous and anthropogenic sources) southern Australian waters. As partof a long term project examining mixotrophy and toxin producing phytoplankton, we conducted a series of experiments to test whether A. catenella utilises humic substances (HS) and how this affects cellular toxin content. Humic utilisation was tested in three different ways: by (1) monitoring growth, N, C, chl a, and toxin content of N-deplete A. catenella (axenic strain CCMP1598) cells in the presence of NO3, NH4, urea and HS; (2) measuring uptake of radioactively labeled humic substances (3HS; isolated from seawater using XAD and subsequently labeled by Amersham); and (3) determining radioactive accumulation in A. catenella using microautoradiography . Cells in NH4 (15 µM) cultures died, but growth in NO3 and HS cultures was similar (0.5 d-1),with urea cultures having the lowest growth (0.2d-1). Toxin concentrations were low (maximum PSP toxins = 0.2 pM), with greater proportions of N-rich C-toxins compared to STX, NEO, and GTX toxins, but there was no apparent difference in toxin content between cells exposed to different N sources. In addition, C (300 - 430 pg) and N (50 - 90 pg) quotas of cells after 14 days were similar in the different N treatments, indicating HS utilisation by A. catenella.. Radioactivity in A.catenella cells increased from 1 - 7 disintegrations per cell [dpc] during the 3HS incubation, with greatest accumulation during the first 24 h (including 1 light and 1 dark period). NO3 adapted cells also accumulated 3HS, with a delay compared to HS adapted cells. Microautoradiography showed that most of the radioactivity was located inside the cell (active uptake), with the remainder being associated with the cell surface (passive adsorption). From these results we conclude that dinoflagellates take up HS directly, that these compounds are a potential source of C and / or N, and that dissolved organic N uptake involves a switch mechanism or simultaneous uptake with inorganic N.