Soybean rust, caused by Phakopsora pachyrhizi Syd., is the most serious soybean disease in Thailand. The commonly grown varieties, S.J.1 and S.J.2, are susceptible to this disease. S. J.4, a new variety recently released by the Department of Agriculture, is only considered more tolerant than that of the mentioned varieties.

Littlejohns and Tanner (1976) have used soybean gennination at low temperature (l0°C) as a criterion for selecting for "cold tolerance". It is our contention that this coupled with a more definitive vigor test might prove to be more reliable in evaluating genotypic differences at low germination temperature. The most prominent soybean vigor tests have been classified into five major categories by McDonald (1976): (1) cold test, (2) accelerated aging test, (3) tetrazolium test, (4) respiration test, and (5) conductivity test.

A chimera plant (A75-1165-117) was observed in 1975 in the F2 of a cross of Ames ms1 x 'Clark' homozygous translocation (Table 1). Reciprocal crosses were made with 'Clark 63', using branches from the chimera plant that contained a high percentage of yellow trifoliolates. Selfed seed of the chimera plant (A75-1165-117) and F1 seed from reciprocal crosses were planted in the field in 1976 (Table 2).