Laboratory experiments were conducted to evaluate the acute toxicity of a widely used textile dye namely Reactive Red 120 (RR 120) on certain aquatic species such as Pseudokirchneriella subcapitata (green alga), Lemna gibba (duck weed), Daphnia magna (water flea) and Oncorhynchus mykiss (Rainbow trout). All experiments were performed as per the OECD Guidelines for Testing of Chemicals. The toxicity end points of EC50, LC50, NOEC and LOEC for RR 120 were determined with 95 % confidence limits using TOX STAT version 3.5. The EC50 of RR 120 for green alga, duck weed and water flea are >100.00, 64.34, 10.40 mg L−1, respectively and LC50 for Rainbow trout is 78.84 mg L−1. Based on the results, the test item RR 120 could be classified as non-toxic to green alga, harmful to duck weed and Rainbow trout, toxic to water flea.