Microcalorimetry was applied to study the basal respiration in several soils collected in Galicia (Northwest Spain) and in the Brazilian Amazon. The microbial activity was recorded microcalorimetrically as power-time lines during 24 h. The soil mass specific heat rate J(Q/S) and the cell specific heat rate J(Q/N) were calculated, and compared to the microbial population of the soil samples and to the number of microorganisms per organic carbon. Results showed an inverse hyperbolic relation between J(Q/N) and number of microorganisms of the samples, and between J(Q/N) and the number of microorganisms per organic carbon. The microcalorimetric indexes of microbial activity were affected by some other soil properties, as percent of carbon, nitrogen, and C/N ratio, as well as by the introduction of agriculture, which affected the microbial population. We believe that the cell specific heat rate can be considered as an index that indicates the efficiency of the energy utilization by soil microorganisms, similarly to the specific respiration activity. The reason of its negative correlation with the microbial density could be attributed to changes in the strategy of the energy utilization by microorganisms in soils. (C) 2003 Elsevier Science B.V. All rights reserved.