At 60 days after planting of yard long bean seedlings, the results showed that the amount of rhizobial population slightly decreased with time for non-managed subplots and the managed subplots of white seeded yard long bean cultivar but largely decreased with the managed subplots of KKU25 yard long bean cultivar and became similar for all treated subplots (those added with native rhizobia) but significantly greater than that of the control subplots (table 3). The results suggested that growth stages of the yard long bean crop plants could possibly have some effectson rhizobial population. For this work, it was foundthat the amount of rhizobial population reached pack period at day 30 after the planting of yard long bean seedlings. Nevertheless, it was observed that the non-managed subplots (no plants) retained a similar amount of rhizobial population as found with those being grown with yard long bean plants of both cultivars. The results indicated that at this stage of crop growth, the yard long bean plants those acting as the hosts of rhizbial may have declined in releasing their exudation substances, hence tremendously affected the growth of rhizbial population. Therefore, it might be inferred that rhizobial population could possibly reach its peak stage coincidentally within the period of the most rapid growth of the yard long bean plants when the crop plants were able to release most of their exudation substances.