The effects of increasing the inclusion of Atriplex amnicola Paul G. Wilson foliage in an oaten (Avena sativa L.) diet fed to sheep were investigated. Six experimental diets were formulated to contain varying amounts [0, 161, 362, 496, 650 and 836 g/kg dry matter (DM)] of A. amnicola, and these diets were fed to individually penned sheep according to a Latin square design. Feed and faecal samples were analysed for crude protein, neutral detergent fibre, acid detergent fibre, acid detergent lignin, and ash contents. Mineral contents and 24 h in vitro gas production were measured in freeze-dried feed samples. As the level of inclusion of A. amnicola increased, digestibility of DM, organic matter and neutral detergent fibre increased (P<0.05), reaching the peak at the 496 g/kg DM inclusion level and then decreasing (P<0.05) thereafter. Acid detergent fibre digestibility followed a similar trend, but peak digesetibility was at the 362 g/kg DM inclusion level. Digestibility of crude protein continued to increase (P<0.05) with increasing inclusion of A. amnicola. Gas production (24 h) from A. amnicola was lower (P<0.05) than that from oaten chaff (24.4 vs 45.3 mL/200 mg DM) and also had lower metabolisable energy content (in vitro ME, 6.3 vs 8.6 MJ/kg DM). As the level of inclusion of A. amnicola in the diet increased, gas production and in vitro metabolisable energy content decreased (P<0.05). The 496 g/kg DM inclusion level provided the optimum N:energy ratio for fermentation of dietary fibre. At this level of inclusion the diet was rich in Na, Ca, P, Mg, K, S, Mn and Zn. High inclusion levels improved the crude protein value of the diet but compromised fibre digestiblity.