AbstractInvestigations with the mycelium forming bacteriumStreptomyces hygroscopicusIMET JA 6599 in glucose‐limited chemostat cultures gave a maintenance coefficient ofm= 0.031 h−1. This low maintenance coefficient corresponds to those of moulds (RIGHELATOet al.1971).With a simple model structured in active and inactive biomass (x1andx2) it was tried to explain the low maintenance coefficient ofS. hygroscopicusin relation to nonfilamentous growing microorganisms. The model contains the transition ratek, which describes the transition of active biomass in inactive one and the decay rate β, which considers the decay of hyphae. The model was used to study the influence of β on the maintenance coefficient and it was shown that maintenance metabolism can be simulated by the parameter β