InMyanmar,the ricemutantvariety‘Shwewartun’ was developed and released in 1975 after seed irradiation of ‘IR5’ in 1970. The improvement in grain quality, seed yield and early maturity of the mutant compared to its parent variety, led to its large-scale planting. Between 1989–93, it covered annually more than 0.8 million ha – 17% of the 4.8 million ha area under rice in Myanmar (Myanmar Agricultural Service, Planning and Project Division, Ministry of Agriculture,Yangon,1995).