The effect of supplementation on the feeding and ruminating behaviour of sheep fed on low quality roughage was assessed at the Shimane University Animal Farm, Japan, in 1993. Four Japanese Corriedale wethers were used for four periods in a 4×4 Latin square design. The initial 10 days of each period were for adaptation by the sheep to their new environment and diets, followed by 5 and 7 days for rumination studies and digestibility trials, respectively. The four diets were: rice straw alone (RSA), rice straw+soyabean meal (RSS), barley straw+molasses meal (BSM) and barley straw+molasses meal+soyabean meal (BSS). Animals on the RSA and BSM diets spent less time eating (305 and 278 min/day respectively) and ate at a slower rate (1·8 g DM/min) than those on the RSS and BSS diets. Although there were no significant differences between treatments (P>0·01) in the number of rumination periods per day, animals on the RSA and BSM diets spent a longer time ruminating per 100 g NDF intake and exhibited slower chewing rates than those on the other diets. Cyclic rate and rumination indexes were both higher for RSA than for other treatment diets. The results show that sheep on the RSA and BSM diets spent less time eating and that those on RSA spent significantly (P<0·05) more time ruminating than those on supplemented diets. Animals on RSA and RSS generally regurgitated more boli but there was no significant difference (P>0·05) in chewing rates between the diets. However, generally, animals on RSA, in addition to regurgitating more boli, also had lower chewing rates, indicating more efficient rumination activity. The judicious use of small amounts of readily degradable nutrients, especially protein, will improve the rumination behaviour of sheep fed on low quality roughage diets, most probably through an improvement in microbial activity. This will result in a higher fibre digestion rate and an increase in the production of ruminal fermentation by-products, compensating for the loss in rumination efficiency.