Bacteria and fungi, the major sources of microorganisms found in soil, play an essential role in nutrient degradations. Both kinds of microorganisms were quantitatively evaluated to provide a correlation between thermal effects and the respective number of microorganisms in the system. The microorganisms were quantified in agar plate counts and the thermal effect was calorimetrically determined in soil samples. The experiments were performed on tropical soils: a Rhodic eutrudox (R), a Typic eutrudox (V) and a Quartzipsamments (Q) from Brazil. The soils were amended with a range of organic materials: 25% of cattle manure, or municipal refuse compost, or earthworm casts, or 23 mug of an agrochemical, trifluralin (doses of 1.25 kg ha(-1)) during incubations of 85 days. The results of simultaneous application of the two methods to measure microbial activity showed the correlation with r = 0.8181 and P = 0.0131 for bacteria and r = 0.8134 and P = 0.014 for fungi, over the period studied. The microbial activity decreased in the order: cattle manure, earthworm casts, municipal refuse compost and trifluralin.