Previously, it was documented that liver peroxisomes display seasonal size changes in the adult Salmo trutta fario, especially in females (and negatively correlated with ovary maturation). It was then hypothesized that decreases in peroxisome size could be paralleled by changes in peroxisomal beta-oxidation and estradiol catabolism actions. The 17beta-hydroxysteroid dehydrogenase 4 has been portrayed as playing an important role in both processes. To elucidate its function in the described peroxisomal pattern, we isolated the cDNA and predicted the protein sequence of the enzyme in that species. The seasonal gene expression pattern in both genders was addressed through quantitative PCR. Fish sampling was in post-spawning period, early and advanced gonad maturation, and pre-spawning. Males did not vary seasonally. As to females, a seasonal pattern was evidenced according to our previous hypothesis. We suggest that the decreased levels observed during vitellogenesis are related to lipid needs for ovary maturation, and, additionally, with the need of modulating estradiol titers.