Free amino acid contents in skin mucus extracts were analysed in sexually undinerentiated yellow eels, female yellow eels and male and female silver eels trapped at the same time from the same freshwater location. Total free amino acid contents in skin mucus were similar in the three groups of sexually differentiated eels (16.2–18.2μM/g dry weight). In sexually undinerentiated yellow eels, total content was higher (39.6μM/g dry weight). Free amino acid composition of skin mucus from male and female silver eels were similar. Compared with the two groups of yellow eels, main differences were found in taurine and non-essential amino acid relative contents. Taurine, glycine and alanine concentrations in skin mucus decreased as the level of sexual maturity of the eel increased. Possible influences of nutritional and hormonal factors on the free amino acid content of eel skin mucus are discussed. Potential consequences in osmoregulatory processes and intraspecific chemocommunication of the eel are also considered.