The dinoflagellate Peridinium aciculiferum forms blooms underneath the ice in temperate lakes and has previously been shown to have an allelopathic effect on a natural competitor as well as being haemolytic. Hence, we investigated whether P. aciculiferum is allelopathic towards a wide range of different freshwater phytoplankton species. We also tested the hypothesis that nutrient (N and P) deficiency enhances its allelopathic effect. In addition, we explored how target cell density affected the extent of the allelopathic effect. Our findings showed that P. aciculiferum had a negative effect (mortality through lyses) on Synura petersenii (Chrysophyceae), Peridinium inconspicuum (Dinophyceae), Cyclotella sp. (Bacillariophyceae), Cryptomonas sp. and Rhodomonas lacustris (Cryptophyceae). Only Chlamydomonas reinhardtii (Chlorophyceae) cells were not negatively affected. In semi-continuously grown nutrient deficient cultures, we detected no allelopathic activity, in contrast to the findings for batch cultures. Finally, we found that, at donor-target cell ratios above 6:1, the algicidal effect of P. aciculiferum did not increase. We concluded that the allelopathic activity of P. aciculiferum has a wide span of action due to either a single chemical or a cocktail of allelochemicals, but that allelopathic activity is highly variable depending on growth conditions.