An experiment with 14 duodenal cannuled goats was undertaken to determine the effects of forage source and of forage level on lipid content and fatty acid (FA) composition of microbial lipids. Lucerne hay (LH) or maize stover (MS) was given chopped to goats in a mixed diet with concentrate at three forage-concentrate levels (100 or 70 or 40 per cent, on the DM basis). The lipid content and the FA composition were determined from the microbial cell mass taken in the duodenal fluid. The total lipid content of the microbial cell mass varied from 10.2 to 20.2 per cent (per DM) of which about half were FA. This lipid content and FA content increased with proportion of concentrate in the diet. The even-straight saturated FA content was higher with LH than with MS. It increased from 65.8 to 81.5 per cent when the forage percentage of the diet decreased from 100 to 40 per cent. The branched-chain FA content and the odd FA content strongly increased with percentage of forage in the diet.