A study was made of nitrogen fixation by bacteria in association with crotalaria (Crotalaria juncea L.) and velvet bean (Stizolobium deeringianum Bart). The experiment was performed according to the Mitscherlich pot technique. Three replications of each of the following treatment were made : 1) check ; 2) NPK ; 3) NPK + limestone ; 4) NPK + inoculation ; 5) NPK + limestone + inoculation ; 6) PK + inoculation ; 7) PK + limestone + inoculation. The amount of nitrogen, phosphorus, potassium, and limestone added to the pots that received these elements were as follows : 2.0 g of N in the form of NH4 NO3 ; 2.2 g of P2O5 as K2HPO4 ; 3.0 g of K2O as K2 HPO4 and KCl; 15 g of a dolomitic limestone in order to increase base saturation in the soil to 60 per cent. Inoculation of the seeds was made with a Rhizobium from the cowpea group. The plants were allowed to grow for two months and were then cut, dried at 60° C, and weighed. The roots were dug, washed carefully, and photographed to show the differences in nodule formation (figures 1 and 2). Nitrogen was determined separately in the stems with leaves and in the roots. This element was also determined in soil samples from treatments n.° 6 and n.° 7 prior to planting and after the plants had been removed from the pots. The data obtained indicated that nitrogen had not been absorbed from the soil. The amount of nitrogen fixed per pot by the nodule bacteria is given below : Treatment Nitrogen fixed by Crotalaria Velvet bean (g) (g) N.º 6 (PK + limestone) ------------------------------------------------------------- 0,52 1,18 N.° 7 (PK + limestone + inoculation) -------------------------------------------- 0,74 1,20