Environmental risk assessment of pesticides often uses the Risk Quotient (RQ) method to characterize risk quantitatively. Toxicological effects of pesticides (for 5 fungicides, 4 herbicides and 6 insecticides formulations) were assessed on the green alga Pseudokirchneriella subcapitata. For each tested pesticide, 96-h acute toxicity test was conducted and toxicity end-points and RQ values based on refinement of exposure concentrations were evaluated. Risk presumptions were presented along with the corresponding level of concern (LOC). Toxicity data indicated differential numerical ranking of pesticides within each pesticide category. For fungicides; mancozeb was found to be the most toxic and difenoconazole was the least toxic (3006 and 93 fold of toxicity; respectively). As related to herbicides, metribuzin was the most effective (1349 fold of toxicity), while cryomazine was practically non-toxic. Moreover, abamectin proved to be the extremely toxic insecticide (5511 fold of toxicity).  Numerical ranking of toxicity was significantly correlated with the RQ ranking in most cases and depends on the nature of the pesticide, bioavailability and exposure concentrations. Associated risk presumptions proved that abamectin, lambda-cyhalothrin, chlorfluazuron and mancozeb pose the worst case acute risk to non-target organisms in aquatic ecosystem and consequently a regulatory action may be warranted in addition to restricted use classification (RQs=27.769; 21.568; 14.833 and 11.678; respectively exceeding LOC). Based on RQ values, risk presumption due to exposure of P. subcapitata toall tested insecticides represents the highly acute potential risk (RQ= 2.988- 27.769 exceeding LOC). Fungicides are also presumed to pose a high risk on aquatic ecosystem but in a trend lower than insecticides. Among tested herbicides, only metribuzin and thiobencarb are presumed to pose a high acute risk on aquatic ecosystem (RQ=2.306 and 1.167; respectively exceeding LOC). The results support the hypothesis that numerical ranking of RQs can be used for the purpose of comparing potential ecological risks and such data may add to the pesticide existing database concerning the potential adverse effects of pesticides on aquatic ecosystem.