Two experiments were conducted to examine the changes in the fatty acid (FA) composition of mixed ruminal microbes (MRM) from sheep fed various levels of dietary forage and soybean oil (SBO). In Experiment 1, diets included five ratios of forage to concentrate. Increased dietary forage did not change MRM concentrations of 18:1(trans-11) and 18:2 (P > 0. 10), but increased 18:3 (P < 0.0 1) and cis-9, trans-11 conjugated linoleic acid (CLA) (P < 0.0 1). In Experiment 2, SBO was added to the diets at 0%, 3.2%, 6.3%, or 9.4% of dietary DM. Increasing dietary SBO resulted in linear increases (P < 0.01) in 18:1(trans-11)and 18:1(cis-9), but linear decreases (P < 0.01) in 18:2 of MRM. It was concluded that FA composition of MRM was affected by diet. Additionally, MRM of sheep fed the diet containing 18.4% forage and 9.4% SBO contained the greatest individual and total FA concentrations. (c) 2007 Elsevier Ltd. All rights reserved.