The population size of endophytic nitrogen fixing or diazotrophic bacteria was determined for three
varieties of cultivated rice and four populations of wild rice. The dynamics of the bacterial populations from
both types of rice was estimated at different stages of plant growth. The number of diazotrophic bacteria in
roots, stems and leaves of all varieties of cultivated rice increased with ageing of the plants to a maximum at
the heading stage (60 days after transplanting). However, nitrogenase activity could not be detected in
bacterial isolates from cultivated rice. The highest bacterial population (5.25 × 106
 per gram fresh material)
was found in the roots of Oryza rufipogon, and this population showed the highest nitrogenase activity. In
wild rice genotypes the populations of endophytic diazotrophs were stable, with the higher population in O.
rufipogon as compared to O. rufipogon (18883) and O. nivara (18852). Cultures of diazotrophic bacteria were
isolated and characterized as species of the genera Azospirillum, Herbaspirillum, Beijerinckia and Pseudomonas.
All isolates were Gram negative and motile, and produced both pectinase and cellulase. Optimum growth
and nitrogen fixation activity for Azospirillum sp. was recorded at 30-35 °C and pH 6.0-7.0, for Beijerinckia
sp. at 25-30 °C and pH 6.0-7.5, and for Herbarspirillum sp. at 30-35 °C and pH 6.0-7.5.