During continuous treatment of salad-oil-manufacturing wastewater using yeast isolates, a large loss of biomass was observed, which subsequently reduced the treatment efficiency. The correlation between biomass washout and loss of species was monitored using the microbiological characteristics of the effluent and the aeration tank. Of the five yeast species, only Candida tropicalis ultimately remained in the aeration tank possibly because it had the best settleability. Addition of nitrogen to the final effluent stimulated the activity of this yeast, which resulted in a treatment efficiency similar to that of the mixed yeast system.