In the enrichment culture of the sulfur-oxidizing bacteria collected from an acid mine drainage, we have confirmed protozoa that belongs to Bodo edax. This paper describes an interaction between the bacteria and the protozoa in the presence of cupric ion. The SO42- produced by the bacteria from S0 and the cell numbers of the bacteria and the protozoa were measured as a function of time in the culture containing Cu2+ of 0, 1, 10, 100 and 1, 000mg dm-3. The results obtained were as follow;
(1) the bacterial oxidation rate and the yield factor of the bacteria growth became small due to the protozoa;
(2) in the cultivation of the bacteria alone, the lag phase of the bacteria growth became long over 1 mg dm-3 of Cu2+ concentration but the growth rate was not affected;
(3) in the culture coexisting the bacteria and the protozoa the bacteria grew, but the protozoa did not grow at Cu2+ concentration of 100mg dm-3;
(4) preincupation in the culture containing Cu2+ gave the ability to resist Cu2+ to the bacteria, but did not to the protozoa.
