Effects of frequency and intensity of defoliation on the symbiotic nitrogen fixation and regrowth of phasey bean (macroptilium lathyroides cv. Murray) plants were studied in a series of glasshouse experiments. Initially, the effects of various culture media on nodulation, vegetative and reproductive growth were compared and the suitability of nitrogen-free solution culture techniques for subsequent experiments was confirmed. Initial and subsequent experiments indicated that in uncut plants, nitrogen accumulated largely in the leaves and stems prior to or during early flowering, but pods were the major sites of nitrogen (and carbon) accumulation during pod-filling stage, with roots and nodules gaining only 10% to 20% of total nitrogen or carbon fixed by plants. At this stage nitrogen fixation declined sharply despite increases in nodule number and nodule dry weight. Regrowth responded markedly to height and frequency of defoliation. Under moderately frequent or infrequent defoliation, height of cut had very little effect on regrowth. However, a tall leafy stubble was always necessary for rapid recovery with frequent defoliation. Maximum dry matter production was obtained with early-flowering defoliation, which coincided with the period of active nodulation and rapid rate of nitrogen fixation. However, in a later experiment high level cutting (node 10) that retained more leaves (and some flowers and pods) markedly reduced regrowth, compared with low (node 5, no flowers retained) defoliation although nitrogen fixation, nodulation and root growth were generally greater following higher cutting. The results suggested that in plants cut at high level, the competitive strength of pods for carbon and nitrogen was initially greater than that of new shoots. Within 2 to 3 hours of defoliation, l4c- labelled assimilate translocation to roots and nodules, and nitrogenase activity decreased markedly relative to the uncut control plants. The extent of this decrease was positively related to the severity of defoliation. Approximately 10 to 15 days after high or low level cutting 14c-labelled assimilate supply to roots and nodules increased rapidly relative to that in the uncut control, followed by corresponding increases in nitrogenase activity and carbon assimilation. In plants cut at node 8 with leaves and flowers retained, 14c-­ labelled assimilate supply to nodules and nitrogenase activity declined significantly during pod-filling, but increased rapidly following pod maturity. Residual leaf removal following high (node 10 or 8) or low (node 5 or 4) level cutting reduced both nitrogen fixation and regrowth. However, in plants cut at node 8, the detrimental effects of leaf removal were overcome by flower removal. Partial nodule removal (28% of nodules on dry weight basis) generally had no effect on nitrogen fixation or regrowth. Without supplementary nitrogen, flower removal in the presence of stubble leaves increased regrowth by 145% compared with flower retention, an effect associated with substantial increases in the rates of nitrogen fixation and carbon assimilation. Supplementary nitrogen (as NH4N03), applied to cut plants with flowers and residual leaves retained, increased regrowth and total carbon assimilation by 170% and 88% respectively, without affecting the yields of retained pods, but suppressed nitrogcn fixation. Without applied nitrogen high level cutting increased regrowth, compared with low defoliation, only when pods were removed. Regrowth was strongly correlated (r = 0.98***) with nitrogen fixation. Although total soluble carbohydrates (TSC) in stubble diminished sharply following defoliation the level was partially replenished during the period of rapid increase in new top growth. Soluble carbohydrates in the roots showed no initial decline, and were poorly related to the area of leaves remaining on the stubble after cutting. Pod removal (by either low level cutting or by flower removal after high cutting), and applied nitrogen markedly increased TSC accumulation in the roots, but these treatments had no consistent effect on TSC in stubble. The data obtained indicate that reductions in the supply of assimilates to roots and nodules, and low rates of nitrogen fixation following intense defoliation or during pod development are major factors limiting carbon assimilation and regrowth. The results support the hypotheses that: (i) symbiotic nitrogen fixation is largely regulated by the supply of recently manufactured assimilates to nodules. (ii) Developing pods remaining on the tall leafy stubble suppress regrowth by competing with new shoots for symbiotic nitrogen, and by competing with the nodules for carbon, thus suppressing nodule activity.