Linoleic acid-rich sunflower-seed supplements (SSS) were used in two experiments (experiment 1, high-concentrate diets; experiment 2, high-forage diets) to study effects on rumen protozoa and the growth of lambs. Both experiments consisted of four treatments, two with a low-protein diet (120 g/kg) and two with a high-protein diet (160 g/kg). For both diets, one treatment was without (control) and one with the SSS (140 g/kg dietary DM). The lambs were fed ad libitum for 70 and 140 d in experiments 1 and 2, respectively. Thereafter, the digestibility of organic matter (OM), acid-detergent fibre and neutral-detergent fibre were determined for each diet with four lambs, and then all lambs were slaughtered and rumen fluid samples were collected and analysed. The results showed substantial decreases (P < 0.001) or total elimination of protozoa in the rumen fluid of the SSS-receiving lambs. In the first experiment the SSS also decreased (P < 0.05) feed intake, but an increase in average daily gain (P < 0.06) resulted in an improved (P < 0.05) feed:gain ratio. Also, the SSS increased (P < 0.05) the digestibility of fibre. In the second experiment the SSS decreased (P < 0.05) the OM digestibility, feed intake and growth of lambs. It was concluded that the use of sunflower-seed supplementation in high-concentrate diets of ruminants reduces rumen fauna, resulting in savings on dietary protein supplements and an increased digestion of feed.