Prymnesium sp. blooms have caused large negative inpacts on ecosystems and aquaculture in both brackish and marine waters. Recently, local blooms of this genus have been observed in the Baltic Sea area. It is currently under question if the high natural organic load into the Baltic Sea basin might partially explain the occurance of these toxic blooms. The hypothesis that P. patelliferum , isolated from the Swedish west coast, could use the nitrogen from freshly extracted riverine dissolved organic material (DOM; MW > 1000), either directly or indirectly via bacteria was tested in discontinuously diluted cultures at a dilution rate of 0.3 d-1. Five different ratios of nitrate and riverine dissolved organic nitrogen (DON) (1, 0.75, 0.5, 0.25 and 0) were supplied at a total nitrogen concentration of 20 µM. Axenic as well as non-axenic cultures of the same strain were tested. However, in the course of the experiment, all cultures were infected by bacteria, probably originating from the DOM extracts. Daily determination of cell density revealed that riverine-DON could not be used efficiently. Moreover, the DOM additions caused a small negative effect on P. patelliferum cell number. Possibly, this is caused by nitrate uptake by the bacteria growing on the DOM. A direct negative effect of bacteria growing on the organic material on the growth of P. patelliferum could not be excluded. This last hypothesis is supported by the fact that in batch culture, axenic cultures showed a higher maximum specific growth rate than non-axenic cultures.The toxicity of this strain, measured as hemolytic potential, was independent on the DOM additions to the cultures.