Samples from different occurrences (trenches or borings) of late Precambrian phosphate deposits in Upper Volta were studied. At Kodjari, the phosphorites are fine-grained and clayey; in cores they are calcareous in the lower part but siliceous above. At Arly the phosphorites are coarse and the grains are frequently rimmed by fibrous apatite and recrystallized. The phosphate mineral is a fluorapatite with a low CO2 content, generally <1%, but with an excess of Ca and F. 3 trace elements are present in notable quantity: Sr, Ba and Mn. The primary deposit was formed in a carbonaceous environment with abundant biological activity. Recrystallization is probably due to deep diagenesis. At the surface, weathering produces aluminium phosphates.