The introduction of genetically modified grain and oilseed products at the farm level and resistance for these products by consumer groups have led to segmentation in grain markets. This study explores the implications for market price behavior for a segregated soybean market for genetically modified (GM) and non-GM varieties. A stochastic dynamic simulation model of production and storage is solved, and Monte Carlo simulation procedures are used to examine price behavior between GM and non-GM soybeans. The results suggest important differences in price behavior between GM and non-GM soybeans. The results obtained in the model simulations are compared with evidence from the Tokyo Grain Exchange, where non-GM and GM soybean futures contracts have traded simultaneously since May 2000. The evidence from the Tokyo Grain Exchange contracts is largely consistent with the results of the simulation model. Price correlations between the Tokyo Grain Exchange non-GM and GM soybean contracts tended to be similar in magnitude to those found in the simulations.