Clonal variability in exponential growth rate and production of secondary metabolites was determined from clonal isolates of Alexandrium tamarense originating from a single geographical population from the east coast of Scotland. To assess variability in the selected phenotypic characteristics over a wide spectrum, 10 clones were chosen for experimentation from 67 clonal isolates pre-screened for their lytic capacity in a standardized bioassay with the cryptophyte Rhodomonas salina. Specific growth rates (μ) of the 10 clonal isolates ranged from 0.28 to 0.46 d-1 and were significantly different among clones. Cell content (fmol cell-1) and composition (mol%) of paralytic shellfish toxins (PSTs), analyzed by liquid chromatography with fluorescence detection (LC-FD), varied widely among these isolates, with total PST quotas ranging from 20 to 89 fmol cell-1. Except for strain 3, the toxins C1/C2, neosaxitoxin (NEO), saxitoxin (STX), and gonyautoxins-1 and -4 (GTX1/GTX4), were consistently the most relatively abundant, with lesser amounts of GTX2/GTX3 evident among all isolates. Only clone 3 contained >20 mol% of toxin B1, with C1/C2, GTX2/GTX3 and NEO in almost equimolar ratios. Eight of the 10 clones caused cell lysis of both R. salina and the heterotrophic dinoflagellate Oxyrrhis marina, as quantified from the dose-response curves from short-term (24 h) co-incubation bioassays. For two clones, no significant mortality even at high Alexandrium cell concentrations (ca. 104 mL-1) was observed. Allelochemical activity expressed as EC50 values, defined as the Alexandrium cell concentration causing lysis of 50% of target cells, varied by about an order of magnitude and was significantly different among clones. No correlation was observed between growth rate und allelochemical potency (as EC50) indicating that at least under non-limiting growth conditions no obvious growth reducing costs are associated with the production of allelochemically active secondary metabolites. © 2009 Elsevier B.V. All rights reserved.