Members of dinflagellate genus Alexandrium are frequently linked to toxic incidents (i.e. PSP) and ecological impacts and thus are among the best characterized organisms of Harmful Algae Blooms widespread throughout many parts of the world. Some species in Alexandrium spp. (e.g. A.tamarense) produce secondary metabolites such as allelopathic substances, independent of PSP toxins, which can inhibit or even lyse other coexisting protists including both photosynthetic and heterotrophic species and could benefit Alexandrium spp. to overcome its relative low growth rate and/or resource uptake capabilities, which generally is believed to be of special importance under stress and tough environmental conditions. In this study, extracellular substances of Alexandrium tamarense were examined by quantification of allelopathic potency based on a bioassay with the cryptophyte Rhodomonas salina under N-deficient compared to nutrient-sufficient conditions along various nitrate additions and growth phases in batch cultures, and under different N-limitations in semi-continuous culutres. The results indicate that extracellular compounds produced by Alexandrium tamarense play an allelopathic role, however, the production of exuded allelochemicals of Alexandrium tamarense was not stimulated by N-limitation. In contrast, slight but significant differences between non-limited control and N-limited treatments indicate that nitrate-limitation even decreased the lytic potency.