The basis of diet selection for protein in ruminants remains unclear. Tolkamp et al., (1998) proposed a hypothesis to account for the conflicting findings. They proposed that ruminants select their diet on the basis of effective rumen degradable protein (eRDP) rather than for MP yield. As the protein supply of ruminants comes largely from microbial protein, the idea that ruminants select a diet that meets the requirement for eRDP, which is captured and utilised by the microbes to produce microbial protein, is not an unreasonable one. The objective of this experiment was to test the hypothesis that both inadequate and abundant eRDP levels would be avoided by growing sheep when given choices of foods.