The fluoride contents of drinking water, dust, soil, straw, raw barley (ears and stems) and barley grains in Khouribga were compared with those from Beni-Mellal, 90 km east of Khouribga where no fluorosis occurs. Identical fluoride levels were found in water samples from both areas, whereas high amounts of F- were in soil, dust, dried straw and dried barley grown in Khouribga. Equilibrated with water, large phosphate rock particles, probably of francolite, and smaller phosphate particles of clay-contaminated francolite, randomly collected in the factories of Khouribga never contained more than 1.2 parts/10(6) F-. Unwashed samples of straw and whole barley had a higher F- content than washed samples. The marked fluorosis in herbivores of the Khouribga area comes more from the ingestion of dust-contaminated food (unwashed straw and unwashed raw barley) with a high fluoride content than from inhalation of fluoride-containing dust. By contrast, the endemic human fluorosis in the same region is mostly due to inhalation of fluoride-containing phosphate dust, and not to fluoride in the drinking water and only exceptionally to fluoride deposited during storage onto cereals, because grains grown in Khouribga have a low fluoride content.