Y-box proteins are conserved regulatory factors that play a key role in coordinating gene activity with protein synthesis by influencing both the transcription and translation of specific subsets of genes. We report the identification of a novel Y-box gene, DeY1, whose transcripts are found in the testes of sexual planarians. DeY1 is expressed in spermatogonia, spermatocytes and spermatids, while no expression is detected in spermatozoa. No DeY1 transcripts are found in the blastema during regeneration. The subcellular distribution of DeY1 protein was analyzed by electron microscope immunocytochemistry. Immunolabelling was found in the nucleus of spermatogonia, in both the nucleus and the cytoplasm of spermatocytes, and in the cytoplasm of spermatids.