Toxicity and identification of LAS were investigated on rainbow trout (Oncorhynchus mykiss) exposed to LAS at various concentration levels in aquarium. The lethal level of LAS was 12.5 mg/L in single döşe and 13.5 mg/L'in progressive doses. The accumulation of LAS in the organs such as gül, heart, stomach and liver were determined by metachromatic method with azur A and toluidine blue. The LAS absorbed on tissue is colored red in examination on microscope.This is the first record in the literatüre using metachromasy for biological trace of LAS in tissue.