The biomass of seven Chilean volcanic ash derived soils was studied with the purpose of establishing the biomass factor (kc) in these ecosystems. To this end a microorganism count was performed by dish culture, the most abundant species were isolated and cultured. The ratio of fungi, bacteria, and actinomyces turned out to be fairly constant. The microorganism isolated from the soils under study were found to contain an average of 41,8% of carbon as determined by the dry combustion method. When measuring CO2 evolution, a studywas done ofthe mineralization ofthe microorganism mass, added as a substrate by the action of soil microorganisms. Parallel to this a study was separately done of the mineralization of fungi, bacteria and actinomyces isolated from Nueva Braunau soil, in their own soil. The same study was performed using red clay soil (Cudico). With the mineralization results and using the microorganisms ratio formed in the colony count, the kc values were calculated for the two above mentioned soils. For Cudico the kc value was 0.464. For Nueva Braunau it was 0.645. The difference found for the parameter in volcanic-ash soils would suggest that the kc of 0.411 found by Anderson and Domsch (1978) would apply to red clay soils, there is in it a certain degree of variability depending on kind of soil and on biomass.