The ratio of respiration to assimilation of glucose in soil microorganisms is reported to increase as the concentrations of added glucose increase. To explain this, a conceptual model has already been proposed in which the glucose acquired by soil microbes is incorporated into either storage compounds or storage compounds together with structural compounds only if sufficient glucose is available for growth. The model also suggested that glucose incorporation into storage compounds required less respiration and more assimilation than incorporation into structural compounds. We hypothesized that soil microbes start to invest acquired glucose into the synthesis of structural compounds when added glucose exceeds a threshold concentration. To confirm this hypothesis we investigated the patterns of glucose use after the addition of glucose C at 23 to 311% of biomass C, levels that are much lower than those tested in previous studies. Respiration rates did not increase (zero-order types) when added glucose C ranged from 23 to 47% of biomass C, but they did increase (growth-associated types) when glucose C was more than 78% of biomass C. The ratio of respiration to utilized glucose was approximately 20% for the zero-order types, but was higher than 20% and increased as the concentrations of added glucose increased for the growth-associated types. In addition, the substrate-induced respiration rate at 12 h after the glucose addition increased only for the growth-associated types, although chloroform-labile C increased in both types as the concentrations of added glucose increased. These results suggested that structural compounds were synthesized only for the growth-associated types and, thus, there was a threshold concentration of glucose over which structural compounds started to be synthesized in soil microbes and the ratio of respiration to assimilation started to increase.