The aim of this study was to investigate the acute toxicity of mercury (II) chloride (HgCl2.H2O) on Corydoras paleatus (callychthidae). Fish were selected for the bioassay experiments, repeated 3 times. The 96-h LC50 value of mercury (II) chloride on Corydoras paleatus was determined by the "Static Acute Method" and found to be 188 mug/L. Water temperature was regulated to be 22 +/- 1 degreesC. In addition, the behavioral changes at each concentration were observed to be significant for the individual fishes with increasing concentration of the toxic compound. Acute toxicity tests were evaluated using the Probit analysis as statistical method.