Pesticides are the chemicals of increased concern regarding their adverse environmental effects. In particular, the reports on their joint toxicity effects are scarce in the literature. Therefore, this paper describes the experiments on toxicities of four pesticides: alachlor, chlorfenvinphos, diuron, and isoproturon, toward Vibrio fischeri. In particular, the joint toxicity effects for all possible binary combinations of the pesticides were analyzed. The analysis included the application of concentration addition and independent action models at two toxicity levels: EC10 and EC50. The analysis revealed additive behavior between all pesticide pairs. The only exception was isoproturon and chlorfenvinphos whose combination resulted in synergistic toxic activity. The original form of the logistic function was given preference over the linearized form in describing the response–dose relationships of investigated pesticides. •Toxicities were determined toward Vibrio fischeri.•Alachlor, chlorfenvinphos, diuron, and isoproturon were examined.•Joint toxicity effects in binary pesticide-mixtures were analyzed.•Isoproturon and chlorfenvinphos indicated synergistic toxic activity.•Other binary combinations indicated additive behavior.