Soybean mutants resistant to Soybean Crinkle Leaf Soybean Crinkle Leaf (SCL) disease, caused by a virus, is a major disease in Thailand. It was first found in Thailand in 1979 [4]. It is transmitted by whitefly ( Bemisia tabaci ). Since 1998, the disease has been found in most main soybean-producing areas, in all growing seasons, and currently is a major constraint to soybean production [5]. Grain yield of soybean can not be produced if the disease infects at the early stage of growth to flowering. Several insecticides are recommended to control whitefly. However, it is costly and not environmentally friendly.cm9238-54-1(ST) is a soybean promising line, it gave 5-10% higher grain yield than cv Chiang Mai 60. But the line is susceptible to SCL disease. Then seed of the linecm9238-54-1(ST) was irradiated with 200 gray. Pedigree method of selection was used from M2-M4. A number of mutant lines were selected under natural field infections at Lop Buri PPTSC and tested in laboratory at the Department of Agriculture, Bangkok. In laboratory, virus-free whitefly was tranferred to infected soybean plants for a day. Then the disease agent was transmitted by releasing 30-40 whiteflies per plant for two days on M4 seedlings lines. Thirty plants were used per line. The plants showing SCL symptom were observed at 40 days after transmission. Yield trials were conducted in 2005 and 2006 at Lop Buri PPTSC. In the 2005 trials, no insecticide was sprayed from emergence to flowering for free whitefly infestations. Later it was sprayed three times after flowering till full seed development to control pod borers and bugs. In the 2006 trial, insecticide was sprayed nine times from emergence to full seed filling stage to completely control whitefly and other insect pests.