The aim of this study was to assess the effect of deltamethrin on rainbow trout (Oncorhynchus mykiss). Control and experimental group of fish were exposed to Decis EW 50 pesticide preparation (active substance 50g/l of deltamethrin). The acute semistatical toxicity test lasting 96h was performed on rainbow trout juveniles. The 96hLC(50) value of Decis EW 50 was 0.02mg/l. Examination of haematological and biochemical profile and histological tissue examination was performed on 1-2-year-old rainbow trout after 96h of exposure to Decis EW 50 in a concentration of 0.02mg/l. The experimental group showed significantly lower values (p<0.05) of plasma glucose, alanine aminotransferase, cholinesterase and significantly higher (p<0.05) values of erythrocyte count, haemoglobin content, haematocrit and plasma total protein, albumins, ammonia, aspartate aminotransferase, creatinekinase and calcium compared to the control group. The deltamethrin-based Decis EW 50 pesticide preparation was classified among substances strongly toxic for fish.