The mechanical properties of different samples of two thermotropic polybibenzoates (poly(heptamethylene) p,p＇-bibenzoate) (P7MB) and poly[oxybis(trimethylene) p,p＇-bibenzoate] (PDTMB) were analysed at several temperatures and deformation rates. One of the polymers presents a very slow transformation of the mesophase into three-dimensional crystal, so that it can be analysed either in the pure mesophase state or in semicrystalline samples. The stress-strain behaviour of a PDTMB sample where only a mesophase is present is intermediate between that of amorphous and semicrystalline polymers, since low values of the Young modulus and high deformation at break are obtained but the deformation takes places through necking even at temperatures above T-g. Moreover, when these thermotropic polymers are allowed to crystallize, their behaviour is typical of semicrystalline polymers.