Soil nitrogen deficiency is a major factor limiting soybean production. This problem can be alleviated by the use of nitrogen fertilizers which on the other hand adversely affect the environment, are expensive and unaffordable to most peasant farmers. Alternatively, attention is being paid to improving soil nitrogen through the use of environmental friendly biological nitrogen fixation of soybeans in an attempt to develop sustainable cropping systems. There is however inadequate knowledge on estimates of nitrogen fixation by soybean varieties in Kenya. In light of this, growth experiments were designed to investigate biological nitrogen fixation in promiscuous and non promiscuous soybeans. Results from the field experiment showed that the two promiscuous soybean varieties (TGx 1869 and TGx 1893) nodulated better than the non promiscuous Gazelle. Uninoculated Gazelle did not produce any nodule while uninoculated TGx 1869 and TGx 1893 nodulated with indigenous soil rhizobia. Inoculation affected nodulation since there was a significantly higher mean nodule number in inoculated soybeans (highest in TGx 1869 IN – 76) as compared to uninoculated (highest in TGx 1869 – UNC - 21) and(0.0) in nitrogen treated soybeans at podding. Inoculated soybeans had higher shoot dry weight (highest in TGx 1893 IN - 19.30 g) than uninoculated soybeans (highest in TGx 1869 UNC - 13.50 g) .Stover biomass was higher in inoculated soybeans (13.07 g) than in uninoculated treatments (6.91 g). Although there was no significant difference in seed dry weight of inoculated (14.10 g) and uninoculated (8.11 g), inoculated soybeans had higher seed dry weight. The inoculated TGx varieties had higher seed dry weight (TGx 1869 at 12.94 g and TGx 1893 at 14.10 g) than Gazelle (10.12 g). There is need to adopt growing of promiscuous TGx varieties and to exploit biological nitrogen fixation with the view of increasing soybean yields and decreasing overdependence on nitrogen fertilizers for sustainable agriculture.