A F₂ population consisting of 178 plants was derived from the cross between a japonica tropical upland variety Azucena and an indica variety Bala. Azucena had long and thick roots when grown in hydroponics, while Bala had thin and short roots. A partial genetic map containing 71 marker loci on 17 linkage groups was constructed and it covered 1280 cM. Only 15 linkage groups with 65 markers that clearly belonged to 12 chromosomes were displayed.