The five different types of bacteria, Bacteroides, Actinomyces, Capnocytophaga, Streptococcus, Fusobacterium which had frequently been recovered in infected canals, were investigated. The purpose of this study was to investigate the bacteriologic status in the traumatized nonvital teeth, and to investigate the effects of bacteria on the size of the le8ion and on the discoloration of teeth. The canal contents of sxiteen traumatized nonvital teeth were sampled and cultured aerobically and anaerobically for growth in five selective agar plates for five bacterial species. The sizes of the radiolucent areas in periapical films were measured and according to the sizes, the samples were divided into two groups. The discoloration of the teeth was checked and according to the existence of the discoloration, the samples were divided into two groups, also. The difference of bacterial colonial numbers in each group was investigated and the following results were obtained. 1. In traumatized nonvital teeth, all of the samples gave bacterial growth except one case. 2. Streptococcus was isolated in four cases but Bacteroides, Actinomyces, Fusobacterium and Capnocytophaga were not isolated. 3. The number of bacterial colonies was not found to be related the size of the lesion periapical films. 4. The number of bacterial colonies was not found to be related the discoloration of teeth.