The behavior of adventitious bacteria during an acidulocomposting process was quantitatively analyzed in garbage-free trials. The numbers of the added Bacillus subtilis and Pseudomonas putida cells diminished in a first-order manner with t(1/2) values of 0.45 d and 0.79 d, respectively, consistent with the observed stability of the acidulocomposting function. (C) 2012, The Society for Biotechnology, Japan. All rights reserved.