Hybrids between male and female thalli of Griffithsia monilis were obtained from somatic cell fusion and regeneration of the fusion cells. The hybrid cells were fused using a wound-healing process, and the fusion cells developed into morphologically normal mature thalli. The hybrids, however, were abnormal in sexual differentiation showing bisexual and bisexual/mixed-phase reproduction. The bisexual/mixed-phase thalli produced tetrasporangia as well as female procarps and male spermatangia. Tetrasporangia were always produced together with spermatangia on a single axial cell. Tetraspores released from the bisexual/mixed-phase thalli developed into male, female, bisexual or bisexual/mixed-phase thalli. The DNA ploidy level indicated that the nuclear phase of bisexual/mixed-phase thalli was haploid. Previously reported models on mixed-phase reproduction such as nuclear co-action and nuclear fusion could not explain the haploid mixed-phase phenomenon. Therefore, a new model, genetic regulation of sex-determining genes, is proposed in this paper.