Field and laboratory experiments were conducted with the following ten cultivars of rice: Lebonnet, Lemont, Dawn, Katy, Della, IR36, Mars, Brazos, Labelle and Jasmine. For each variety, components of biomass, root porosity and methane emission were observed throughout the entire growing season and yield was determined at harvest. Methane emission differed among cultivars by as much as a factor of 2.4 and resulted in two distinct emission groups. Significant differences were also found for biomass among cultivars, although these differences did not coincide with the differences that were observed for emission among cultivars. Methane emission correlated strongly with aboveground live vegetative biomass within most varieties until heading and among cultivars within emission groups to heading. Emissions showed less correlation with biomass during ripening and may have been affected by other factors within the system at this time. Methane emission appeared to be consistently proportional to grain production among cultivars, when determined per gram biomass, and may be related through processes of carbohydrate partitioning. Root porosity did not appear to be associated with observed differences or trends in methane emission.