Here it is evaluated the relationship between substrate-induced respiration (SIR) and microbial biomass C (C(mic), estimated by chloroform fumigation-extraction) in order to establish SIR as a quick technique to determine microbial biomass in vermicomposting processes. For this, there were designed continuous feeding reactors in which new layers of manure were added sequentially to form an age gradient inside the reactors Six reactors were set up with and without earthworms (three reactors per treatment) In reactors, with and without earthworms, values of C(mic) and SIR ranged from 1690 to 42,900 mu g g(-1) dw and from 43 to 2300 mu g CO(2) h(-1) g(-1) dw, respectively. SIR was significantly related to C(mic) (r = 063, P<0.0001) It is proposed an equation to convert SIR values Into C(mic), SIR (mu g CO(2) h(-1) g(-1) dw) = 25.97 + 004 C(mic) (mu g g(-1) dw) which will be useful for comparison between studies which used different techniques for determining microbial biomass (C) 2010 Elsevier Ltd. All rights reserved