Light interception is a key element of crop production, and improvement of light-interception efficiency is therefore an important aim in breeding programs. Petiole length is known to affect light-intercepting characteristics in soybean, but despite the importance of this finding the genetic and physiological mechanisms of petiole length control have not been fully elucidated. To improve our understanding of petiole length control, we investigated two mutant lines that exhibited short petioles with slightly curly leaves when grown in the field. Crossing tests revealed that the same recessive gene caused this unique form in both lines. Linkage analysis revealed that the mutated locus was located on the short arm of chromosome 11. We tentatively named this locus Chunky after the plant form of the mutants, and we are now trying to identify the gene by using map-based cloning. An interesting peculiarity of these mutants is their sensitivity to environmental conditions. Mutant plants grown in a greenhouse did not exhibit the mutant form: their petiole length was almost the same as that of a wild-type sib line. This phenomenon will be crucial for estimating the gene function of Chunky(t), because it proved that the mutant lines had not lost their ability to extend their petioles. The environmental factors affecting morphogenesis in these mutants are currently unknown. Identification of these factors would help to elucidate the function of Chunky(t).