SummaryWeeds were collected from carrot fields and some other sites at 27 localities in seven counties in Norway during August and September of 1986 and 1987. Leaf and petiole lesions were microscopically examined forMycocentrospora acerina. Viola arvensisandViola tricolorwere commonly observed with the pathogen. The fungus was also registered onFumaria officinalis, Galium aparine, Matricaria inodora, Matricaria matricarioides, Ranunculus repens, Senecio vulgarisandStachys palustris.In inoculation experiments in the greenhouse,M. acerinainfected carrot leaves and 13 of 16 species of weeds. The symptoms on the various susceptible plants showed some variation, but were essentially of the same pattern. The investigation indicates that weeds are common hosts of M.acerina.A list of 23 host weed species is given.