Acute canine distemper virus (CDV)-induced demyelinating lesions were examined with double-labelling immunocytochemistry simultaneously demonstrating CDV antigen and glial fibrillary acidid protein (GFAP) as marker for astrocytes. It was shown that 64% of all astrocytes within the demyelinating lesions were infected and that 95% of all infected cells counted in the lesions were astrocytes. These results suggest that the astrocyte is the main target for CDV and that astroglial infection may play an important role in the mechanism of demyelination.