Whole cottonseed (WCS) is a by-product of the cotton-gin industry. This feedstuff is of significant feeding value for animals. WCS dry matter is high in fat, crude protein and neutral detergent fiber (NRC). The effect of fat on rumen-fermentation characteristics is explained by its detrimental effect on rumen microorganisms (Arieli, 1998). Chalupa et al (1984) found in an in-vitro study that addition of free C18:1, C18:2 and C18:3 to a hay-grain diet decreased the volatile fatty acids (VFA's) concentrations and acetate-to-propionate ratio. In some experiments, WCS had no effect on rumen fermentation. Sklan et al. (1992) found that the acetate:propionate ratio increased with WCS. The aim of this study was to survey the effect of addition of WCS and decreasing dietary CP level on rumen parameters.