To obtain basic information about haploid plants in Citrus, the morphological characteristics and reproductive potential of a haploid plant obtained from the ‘Banpeiyu’ pummelo [C. maxima (Burm.) Merr.] were examined. The leaves, flowers and fruits of the haploid were significantly smaller than those of the ‘Banpeiyu’ pummelo. Although the haploid showed remarkably reduced pollen fertility compared with the ‘Banpeiyu’ pummelo, it produced slightly fertile pollen grains. In crosses with diploid cultivars, when the haploid was the seed parent, there was no seed. But when diploid cultivars were pollinated with pollen of the haploid, diploid progenies were obtained. During female gamete formation, no embryo-sac mother cell was formed in the ovules, and consequently no normal embryo sac was observed at anthesis, owing to female sterility caused by the lack of embryo-sac mother cell (EMC) formation in the haploid. During male gamete formation, however, univalents were not distributed to opposite poles at anaphase I in some dividing cells; instead, they remained near the equatorial plate. The nine univalents then performed mitosis, and the univalents were equally distributed to opposite poles. And at the tetrad stage, dyads were also produced in the haploid at a ratio of 24.7%. These results suggest that fertile pollen grains in the haploid would be dyad derivation, produced by abnormalities in the first meiotic division such as first division restitution (FDR).