This report describes a pair of human oocytes of differing maturities but fused at the zona pellucida, believed to have originated from a binovular follicle. After insemination in vitro, one oocyte fertilized and developed into an expanded blastocyst, whereas the other, which contained a germinal vesicle, degenerated. A biopsy was aspirated from the blastocyst, which subsequently re-expanded but did not hatch.