Olive Oil production is of great economical importance in many Mediterranean countries as well as in the prefecture of Messinia which is the second largest olive oil production area in Greece (35.000-60.000 tn of olive oil per year). However, the vast olive oil waste water quantities that are annually produced in a short period of time consist of a serious environmental problem since up till now there is no treatment system applied and the wastes are being discharged into nearby streams and rivers. More specifically, the wastewaters are spread through canals and small rivers into the major rivers of Messinia during the production period (November-late January) and finally outflow in the marine ecosystem of Messiniakos Gulf. Almost 67% of the active olive oil industries in Messinia operate at a three phase system and the amount of olive oil waste waters that is annually produced reaches 250.000 tn. These dark colored liquid wastes contain large amounts of organic pollutants, a high quantity of phenols, are rich in nitrogen and phosphorus and also contain amounts of trace metals (Fe, Cr, Pb, Cu, Ni, Cd, Zn & Mn). The toxicity of these wastes was evaluated using an acute toxicity bioassay. The toxicity of Olive Oil Mill Waste Waters (OMWW) effluent to aquatic macroinvertebrates in order to assess the quality of freshwaters has not yet been studied extensively (Rosenberg and Resh, 1993). Acute toxicity is the capacity of a substance to cause harmful effects at short term (minutes, hours or days) after direct exposure either oral, by skin contact or by respiration. The aim of this work was to estimate the toxicity of OMWW effluents on stream macroinvertebrates that live in the area by using 24h Lethal Concentration 50 bioassay. This concentration is the concentration in which the 50% of the population of the test organisms dies within 24 hours of direct exposure. The lower the value of the concentration the more toxic is the material.