Cadimium ion-resistant microorganism was isolated from the sludge of wastewater. The physiological, morphological and other cultural data showed that this strain belonged to Citrobacter freudii. A clearcut distinction of growth among nutrient broth, typtic soy broth and synthetic medium was demonstrated. The resistant cells showed only slight mutagenic action. During the growth of bacterial population in resting state, the organisms reduced the initial level of resistance to cadmium ions when they were not kept in contact with cadmium ions in bacteral multiplication. And cadmium ion-resistant and cadmium ion-sensitive strain were found to show equal, lower or higher sensitivity to other heave metals.