The investigations were performed on carp at three stages of development (fry, light commercial fishes and commercial fishes) caught at four times over the period of three years. Mercury was determined by the ASA method using the autoanalyzer AMA-254. The sensitivity was 0.01 ng Hg.g-1. Mercury contents in the muscles was significantly negatively correlated with the weight (age) of fishes. The correlation coefficient was -0.505, n=80. Mean Hg concentration in the muscles of fry, two year old fishes, light commercial and commercial fishes were: 0.070, 0.064, 0.040 and 0.031 mg.kg-1 respectively. It could result from various proportions of natural and commercial feeds in ratios for young and older carp.