Oligodendrocyte progenitors from the adult rodent optic nerve were described in vitro more than a decade ago but their identification in vivo has been more elusive. We have previously traced newly-generated oligodendrocytes remyelinating focal demyelination of adult feline optic nerve back to their origin in surrounding fascicles. Extending the search to normal tissue has now enabled the description of an oligodendrocyte progenitor population in situ. By combining serial 1 mum section immunocytochemistry with electron microscopy of perfusion-fixed tissue, every cell of interest could be characterised. Oligodendrocyte progenitors were readily identifiable in transverse and longitudinal sections dual-stained with antibodies to glutamine synthetase and either S-100 or HNK-1. Oligodendrocyte progenitors of the adult feline optic nerve were mainly located centrally within fascicles, comprised 4% of the total macroglia and had both ultrastructural and immunocytochemical features suggesting roles additional to that of a progenitor cell. The identification and characterisation of oligodendrocyte progenitors in situ in fixed tissue sections should help the understanding of their role in the adult CNS and in disease such as multiple sclerosis.