In this work, the flammability properties of a new rigid rod polymer fibre are examined. This new fibre was called "M5" and is formally poly[2,6-diimidazo[4,5-b:4',5'-e]pyridinylene-1,4(2,5-dihydroxy)phenylene or PIPD. In a first step, characterisation of PIPD (M5) by solid state NMR confirms the chemical composition of the fibre. The investigation of the fire performance of M5 fibers using the cone calorimeter has shown the excellent behaviour of M5 in comparison with poly-p-phenylenediamine-terephtalamide (PPTA). M5 fibre does not burn even in very severe conditions (flashover conditions). It can then be used where high level of safety is required. The investigation of its heat resistance shows that M5 degrades via exothermic reactions of 30% lower energy than PPTA and have a strong pyrolysis resistance which can explain, in part, its unique fire properties. (C) 2003 Kluwer Academic Publishers.