Abstract: This study described pathological features of a spontaneous spermatocytic seminoma in gibel carp Carassius gibelio. Histologically, the neoplasm founded in the abdomen was encapsulated invasive spermatocytic seminoma with an inconspicuous lobular architecture and diffuse fibro-connective septa with lymphocytic infiltration. For its ultrastructure, undifferentiated spermatocytes and fewer supporting cells were the unique features of the neoplasm. Compared with the histological features of normal testis of gibel carp, this neoplasm was identified as a spermatocytic seminoma. The first report of spermatocytic seminoma with lymphocytic infiltration in fish may contribute to the establishment of fish model of spermatocytic seminoma.