Diazotrophic Alcaligenes faecalis strain A15 isolated from rice roots is associated with rice roots and resides in the mucilage layer. On examination by light and electron microscopy, about 10% of the bacteria that accumulated on the surface entered the root cells. Some bacteria were observed to penetrate the cell wall. By using a 10B α-track technique, the presence of bacteria inside the root was also demonstrated after the rice plants were inoculated with A. faecalis. This was further supported by results of an immunofluorescence study. Both single cells and protoplasts were isolated from rice root and its callus. Transmission and scanning electron microscopy showed that A. faecalis grew inside the root cell. Callus, developed from rice root cells inoculated with this bacterium, grew well in nitrogen-free culture medium, while it died within 30 days without inoculation. Nitrogen fixation in the callus was confirmed by 15N tracer methodology and the rate ranged from 18.5 to 35.5 μg N fixed/g dry weight callus per day.Key words: Alcaligenes faecalis, endorhizospheric nitrogen fixation, rice–bacteria symbiont.