Single and joint effects of two antifouling booster biocides, irgarol 1051 (2-methylthio-4-tert-butylamino-6-cyclopropylamino-s-triazine) and diuron (1-(3,4 dichlorophenyl)-3,3 dimethyl urea), their metabolites, M1 (2-methylthio-4-tert-butylamino-s-triazine), DCPMU (1-(3,4-dichlorophenyl)-3 methyl urea), DCPU (1-(3,4 dichlorophenyl urea) and DCA (3,4-dichloroaniline), respectively, as well as copper were examined. Two phytoplanktonic microorganisms, the green alga Dunaliella tertiolecta and the diatom Navicula forcipata were exposed to various concentrations of the aforementioned compounds both alone and in binary mixtures during a period of 96h. Estimation of EC(50) values was performed by daily cell number counting of the tested microorganisms. The toxicity of the six compounds and the metal, applied singly, was found to be, in decreasing order, irgarol 1051>diuron>M1>DCPMU>DCA>Cu>DCPU and irgarol 1051>diuron>M1>DCA for the green alga and the diatom, respectively. Diatoms were found to be more sensitive in the presence of all the tested compounds, except diuron. Co-existence of irgarol 1051 and M1 revealed additive effects on both microorganisms. Same results were observed owing to the joint action of copper with either irgarol 1051 or M1 for almost all the examined mixtures. Combined effects of diuron with its metabolites DCPMU and DCA resulted in synergism in almost all cases, for both species of phytoplankton. On the contrary, antagonistic effects were observed owing to the joint action of copper with either diuron or one of its metabolites.