The importance of the Bcl-2 family proteins in normal vertebrate embryogenesis is being recognized; however, their regulatory mechanism is poorly understood. To elucidate the embryonic roles of Bcl-2 family proteins, we cloned and characterized the first zebrafish Bcl-2 family protein, zfMcl-1a. Zebrafish Mc1-1a shows the highest homology to rat Mc1-1 and contains several conserved BH domains of the Bcl-2 family proteins. It also contains a nuclear localization signal (NLS). Using EGFP reporter analysis, we verified the nuclear localization of zfMcl-1a. Deletion of the NLS resulted in distribution of the fusion protein in the cytoplasm. Northern blot analysis indicated that zfMcl-1a mRNA is 1.5 kb and was expressed in oocytes and throughout embryonic development. Notably, the expression of zfMcl-1a transcript was significantly downregulated during gastrulation. These results suggest that zfMcl-1a is a novel nuclear Bcl-2 family protein and is likely to play an important role in zebrafish oogenesis and embryogenesis.