This study is concerned with an investigation into the relationship between morphology and mechanical properties for poly(p-phenylene benzobisoxazole) (PBO) fibres performed using Raman spectroscopy. The three fibres studied have different modulus values tin the range 180-360 GPa), as a result of different processing conditions leading to different molecular orientation and the presence of density fluctuations along the fibre axis. It is found that the 360 Cpa modulus fibre has a strain-induced Raman band shift value of -12 cm(-1)/% which is the highest reported value for aromatic polymer fibres such as PBO and(l-aramids, The strain-induced Raman band broadening value of this fibre is the lowest among the fibres studied. Compressive strengths for the PBO fibres were also measured and found to be similar (0.29-0.35 GPa). It was also shown that the Raman band broadening becomes less in compression, which is different from the behaviour of carbon fibres and a characteristic feature of PBO fibres. The relationship between the band broadening and fibre morphology is discussed in detail.