The present study assesses acute toxicity of non-ionized ammonia and potassium dichromate using the estuarine benthic copepod Nitokra sp from laboratory cultures. Bioassays with non-ionized ammonia sensitivity were carried out, and the mean. Bioassays with non-ionized ammonia sensitivity were carried out, and the mean of LC(I) 50-96h was 1.7 mg.L-1 .-LC(I) 50-96h was 1.7 mg.L-1. Potassium dichromate was used as reference substance in two distinct periods of exposure and the mean LC(I) 50-48h and LC(I) 50-96h values were 55.61 mg.L-1 and 21.70 mg.L-1 , respectively. The organism showed sensitivity to the toxic agents used and good reproducibility of results, indispensable factors for use in ecotoxicological assays.