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A study was made over 2 years of the nutritive value of the subterranean clover (Trifolium subterraneum L.) cultivars, Trikkala, Larisa and Goulburn, in grazing experiments with Border Leicester x Merino lambs. Liveweight gain was similar for all cultivars in the first year and in the second year was higher on Larisa (153 g/day) compared with Goulburn and Trikkala (131 and 103 g/day, respectively). Wool growth was not significantly different between cultivars. Both the digestibility and water-soluble carbohydrate content of the petiole and stem plant fraction were significantly (P<0.05) higher than those of the leaf fraction, even though the leaf had a lower level of cell wall organic matter. The ratio of lignin to cell wall organic matter was also higher in the leaf fraction. Samples obtained via oesophageal fistula showed that petioles comprised most of the dry matter in the diet and, thus, were the major contributor to energy supply. Differences were found in the degradability characteristics of the 3 cultivars which led to significant (P<0.05) differences in the concentration of rumen degradable dry matter and effective rumen degradable protein (ERDP). During the vegetative and early flowering stages, the concentration of ERDP exceeded the supply of fermentable metabolisable energy required for microbial protein synthesis. In mature clover, the concentration of ERDP was low and limiting microbial protein synthesis. The degradation characteristics of protein were highly correlated with liveweight gain (r2 = 0.90) and wool growth (r2 = 0.70). It is suggested that increasing the petiole : leaf ratio could increase the efficiency of utilisation of subterranean clover diets.