The compressive yield behaviour of poly (2,2’-m-phenylene-5,5’-bibenzimidazole) (PBI) has been studied over a wide range of temperatures. The tensile behaviour was also studied under superimposed hydrostatic pressure. In both cases wet and dry samples were examined and the results revealed the considerable effects of moisture on the mechanical properties of PBI. The results of all these tests show that PBI has a remarkably high shear yield stress at room temperature. This results in a very high compressive yield stress and a very high tensile yield stress observed under superimposed hydrostatic pressure when brittle failure from surface flaws is prevented. It is concluded on the basis of quantitative analysis that the yield mechanism in PBI at room temperature is initiation controlled, as in a metal or ceramic, rather than a velocity controlled, thermally activated, viscoelastic process which is generally considered applicable in polymers.