Five cases of bovine udder aspergillosis diagnosed clinically and confirmed by repeated demonstration of fungal hyphae in KOH preparations and isolation of the organism from milk sediments are described. In one case the fungus was identified as Aspergillus nidulans, in the other 4 cases as A. fumigatus. The most pronounced features in altered udders were multiple abscesses with distinctive colonies of fungal hyphae in the centres, embedded in cell detritus, neutrophils and macrophages. These purulent centres were surrounded by varying amounts of granulation tissue. Two cases revealed purulent galactophoritis suggesting an ascending infection through the lactiferous ducts. In one case no changes of the milk ducts were observed. In this case haematogenous infection of the udder cannot be excluded.