A case of bicephaly in embryos of the blue shark Prionace glauca in the Mexican Pacific Ocean is described. Malformed female shark embryos were found in pregnant females caught in the Gulf of California (Punta Arenas) and off the western coast of Baja California Sur, Mexico (San Lázaro). Abnormal sharks showed a symmetric bicephaly that could be caused by the high number of embryos found in the uterus of the blue shark, which is the most fecund species of shark in the world. The abnormality probably began during the embryonic development..