Microbial growth rate was estimated by multiplying15N immobilization by an estimated microbial C:N ratio. This growth rate, in combination with measurements of respiration, was used to calculate growth efficiency. Growth rates and efficiencies were calculated for grassland and cultivated soils of three textures. Calculated efficiencies (Yc), assuming a microbial C: N ratio of 7, ranged from 32 to 54. Cultivated soils tended to have higher Yc values than did grassland soils. This calculation depends on several hard-to-verify assumptions, but yields numbers that should be of great interest in comparative studies.