The citrus brown mite Eutetranychus orientalis is a common spider mite, which is a harmful pest in citrus orchards in Egypt. The toxic activities of some microalgae isolated from Egyptian soils and water are studied against E. orientalis. The toxicity of whole cultures (culturing broth and cell) of seven algal strains was tested against E. orientalis. Three strains out of the tested seven strains showed higher toxicity with mortality percentage reached up to 99 % after 72 h recorded against adult individuals of E. orientalis with a strain of Chlorella minutissima. The toxic activity was also tested against immature individuals and eggs of E. orientalis. The highest mortality percent against immature individuals was observed with C. oocystoids. Eggs were not affected with any of the tested algal strains. When cells and culturing broth were tested separately the activity was higher in culturing broth which means that this activity is due to some extracellular metabolites secreted by algae in their culturing medium. When the organic solvent n-butanol was used to extract the active constituents from the whole cultures (culturing broth and cell), the activity was found to be higher in aqueous phase compared to the butanolic phase.