Faecal samples taken from eight underweight, approximately 5-week-old broiler chickens in a poultry abattoir were investigated for microsporidial infections by light microscopy, electron microscopy, and PCR. In two of six chickens, which were suspected of being infected with microsporidia by light microscopy, Enterocytozoon bieneusi (genotype 'J') was detected by PCR and DNA sequencing, and in one of the two PCR-positive samples by extensive electron microscopy. This is the first time that E. bieneusi has been detected in chickens, i.e. in a non-mammalian species.