The influence of salinity and a number of nitrogen (N) and phosphorus (P) levels on growth and haemolytic activity of the harmful algal bloom (HAB) species Fibrocapsa japonica were studied simultaneously. The F japonica strain studied had a euryhaline growth profile with an optimum growth rate at 26 psu. Measurements of the minimum cell quota of N and P revealed an optimum N:P ratio of 24.5 and a relatively high requirement of nutrients (q(0)N = 7.1 pmol cell(-1); q(0)P = 0.29 pmol cell(-1)). These characteristics fit the habitat of this species, among which is the P-controlled eutrophied Dutch coastal waters. Salinity had a significant effect on the growth rate of F japonica and on its haemolytic activity, even on the basis of equal cell volume. The highest haemolytic activity was found at 16 psu, the lowest salinity condition tested. The EC50 values that were expressed on a per cell basis revealed that F. japonica had a high haemolytic activity when compared with other HAB species. Under N limitation, cells of F. japonica were less haemolytic than cells not limited for N, but this could be caused by a slight decrease in cell volume only. Remarkably, cells not limited for N appeared to form aggregates, which in combination with the elevated levels of haemolytic compounds per cell may result in obstruction of gills, causing fish kills during F japonica blooms. So, despite the absence of brevetoxin in our strain of F. japonica, this strain may be lethal to fish due to its haemolytic activity.