Bulk densities of surface fuels divided into three strata were determined for dominant fuel groups to aid in describing compactness and horizontal continuity for fire behavior modeling. Dominant fuel groups were defined as horizontally distributed fuels having recognizably similar physical properties. Eleven vegetation types of varying overstory composition and understory structure were investigated in western Montana and northern Idaho. Bulk density varied substantially and averaged 10.0 kg/m³ a in litter-type dominant fuel groups. It averaged 3.9 kg/m³ and varied slightly in mixed and upright dominant fuel groups. Determination of bulk density for vegetation types can replace the need to measure fuel depth in Rothermel's (1972) fire spread model. For modeling fire behavior in nonuniform fuels, two to three dominant fuel groups were optimum because they provided almost as much precision as six groups and considerably more than one group. Incorporation of dominant fuel groups in fuel modeling can increase precision and perhaps accuracy of predicted fire behavior and provide flexibility to users in classifying fuels.