The development of soybean [Glycine max (L.) Merr.] cultivars for natto production relies on the selection of seed traits that influence natto quality. The objectives of this study were to evaluate the relationship of 11 seed traits to six natto quality traits and to determine the effects of genotype and environment on the traits. Sixteen small-seeded lines and cultivars were grown in replicated tests at three Iowa locations during two years. The natto quality traits evaluated for each plot were water absorption, water loss after steaming, hardness of steamed seed and natto, and darkness of steamed seeds and natto. The seed traits evaluated were total sugar, free sugar, sucrose, raffinose, stachyose, protein, oil, fiber, protein + oil, protein + oil + fiber, and seed size. To efficiently evaluate total sugar content in this study, a rapid method for total sugar determination by acid hydrolysis was developed. Significant differences among genotypes were observed for all traits, except fiber content. There were significant differences between years or among locations for all traits, except water loss and stachyose. All seed traits, except stachyose, oil, and seed size had significant associations with natto quality traits. Protein + oil content was significantly correlated with natto quality traits and would be considered a useful selection criterion in a breeding program. There was a significant phenotypic correlation of --0.90 between total sugar and protein + oil. To further assess the relationship of total sugar content to other seed traits, 23 diverse soybean cultivars grown at eight Iowa locations in 1998 were evaluated for total sugar, protein, oil, fiber, protein + oil, and protein + oil + fiber. Total sugar content was highly correlated with protein + oil (--0.81). The analysis of protein + oil by near-infrared reflectance should be a useful predictor of total sugar content.