Young cashmere bucks (n = 113; offspring of mating of domesticated mainland feral does with commercial mainland, Bernier, and Faure Island bucks) were offered a high protein-high energy pelleted diet of lucerne chaff, lupin seed, and barley grain [crude protein, 202 kg; dry matter (DM) solubility, 75.0%] or a low protein-low energy diet of oaten chaff and oat grain (crude protein, 71 kg; DM solubility, 59.6%) ad libitum in groups (n = 4-8) from 14 January to 17 December 1987. Intakes and liveweight gains were measured for January-April, May-July, and August-December. Total fibre and cashmere production (weight, fibre diameter, yield) were recorded at shearing in June, October, and December. Faure bucks produced 34 and 64% more (P<0.05) cashmere on the high, than the low, quality diet at shearings in June and December, respectively; Bernier bucks produced twice as much (P<0.05) cashmere on the high quality diet at the October shearing. Mainland bucks produced similar amounts of cashmere on both diets at each shearing. The fibre produced on the high quality diet had a lower yield of cashmere and the cashmere had a greater diameter at shearings in June (26.0 v. 41.1% w/w, 16.7 v. 15.8 pm; both P<0.001) and December (22.1 v. 27.0% w/w, P<0.05; 16.9 v. 16.4 pm, P<0.01). The cashmere produced by Faure bucks was, on average, 1.0 pm coarser than that of the mainland and Bernier bucks at each shearing. Liveweights of bucks followed a seasonal pattern and increased at a faster rate in January-April and August-December than in May-July. Liveweight gains adjusted for body sizes were greater (P<0.001) for bucks on the high, than the low, quality diet in January-April (2.85 v. 1.71 g/kg LW0.75.day) and, conversely, lower (P<0.05) in August-December (1.49 v. 1.69 g/kg LW0.75.day) . Liveweight gains of all 3 genotypes adjusted for body sizes were markedly lower (P<0.05) in May-July, irrespective of diet (0.75 and 0.55 g/kg LW0.75.day for the high and low quality diet, respectively), with the exception of Bernier bucks on the high quality diet (1.67 g/kg LW0.75.day). The lower liveweight gains were not associated with decreases in the intakes of the diets. These results indicate that diet quality affects cashmere production and growth of Western Australian cashmere goats.