A simple and effective method to estimate the specific growth rate estimation has been developed based on the observation of time changes in the agitation speed in dissolved oxygen(DO)-stat cultures of Brevibacterium ketoglutamicum. The estimation was compared with that using carbon dioxide evolution rate (CER). Estimated values of specific growth rates by both methods agreed well with the data directly calculated from cell concentration change although the use of agitation speed gave a slightly better result than CER.