---

The aim of this study was to investigate the effect of fish size and seasonality on the epidermis structure and protein profile of the skin mucus of Rainbow trout. Skin mucus samples were collected over three consecutive season periods including late winter, mid spring and early autumn. In each season, sampling was performed separately from three different weight groups including 2-20 gr fish (W1), 100-200 gr fish (W2) and 400-600 gr fish (W3). The results of histological analysis showed that goblet cell density and epidermal thickness increased with increasing temperature from winter to autumn in all weight groups. In juvenile (W1 fish), the size of goblet cells was not affected by the season, while in larger fishes (W2 and W3), the size decreased with increasing temperature in the autumn. There was a positive and significant relationship between fish size and density of goblet cells. Biochemical analysis of skin mucus showed that protein concentration increased simultaneously with an increase in goblet cell density. The protein profiles of rainbow trout mucus showed seasonal variations in all three weight groups. In autumn, the small proteins with molecular weights less than 35 kDa were more prominent, while these proteins were not observed in the winter. There were no significant differences in the mucus protein profile among different weight groups