Experiments are described in which a study was made of the top growth, root weight, nodule number, and type of nodules of white clover growing in a grass sward in the field. Soil cores were taken and the clover root-system examined after removal of soil by washing.The most important factor governing the total number of nodules and the number of large (2 mm. or more in length) nodules under the sward was the amount of clover root-material present.Nitrogenous fertilisers and the return of dung and urine by grazing sheep reduced the amount of clover root-weight and consequently the number of nodules found under the pasture. 36 lb. of nitrogen applied in the early spring of 1956 had a beneficial effect on both clover top-growth and nodulation. 80-100 lb. of nitrogen per acre had no effect on the number of nodules per gram of root. However, the application of 200 lb. of nitrogen per acre caused a significant decrease in the number of large nodules F>er gram of root.The numbers of large nodules present reached a midsummer peak where nitrogen was not applied.During the autumn many nodules showed part green and part pink colouration or were completely green or brown, an indication of a decline in nitrogen-fixing ability. Decaying nodules increased considerably in number when the legume was being suppressed by nitrogen application and top growth was poor. The practical application of these results is discussed and methods of maximising the benefit from clover and fertiliser nitrogen are suggested.