‘Ruby Red’, the first commercial grapefruit cultivar with red-pigmented fruit and blushed peel, originated from a spontaneous mutation discovered in Texas in 1929. However, its flesh color faded as the harvest season progressed, and the juice colour was not accepted. A redder grapefruit was desired. Seeds and bud wood were irradiated and two cultivars were obtained. ‘Star Ruby’, released in Texas in 1970, was obtained by thermal neutron irradiation of ‘Hudson’ seeds. It is seedless, red flesh and has gained wide acceptance, but its yield is variable. ‘Rio Red’, released in 1984 in Texas, was derived from irradiated bud wood of Star Ruby with thermal neutrons(Hensz, 1991). It has red flesh and a good yield (Spiegel-Roy, 1990).Rio Red became the preferredcultivar in Texas and in other grapefruit growing areas. The fruits of both cultivars are sold under the trademark‘Rio Star’. In Texas, Rio Star grapefruit is currently grown on 7,300 ha, which is 75% of the grapefruit production area in Texas (Sauls, 1999). The development of the two radiation induced mutant cultivars is considered as the most significant break through in grapefruit growing in Texas since the discovery of Ruby Red in 1929.