The sedimentary iron ores of Britain all contain small amounts of phosphorus which is generally present in the colloform mineral collophane. In a rather uncommon class of oolitic ironstones recently described from the Yorkshire coalfield, a crystalline phosphate was found which has now been identified as francolite and forms the subject of this paper. These oolitic ironstones occur in the immediate roofs of coal seams, are of freshwater origin, and consist of ooliths of kaolinite and isotropic clay in a groundmass of chalybite. During the later stages of their diagenesis, francolite, associated with quartz, calcite, pyrite and blonde, and much kaolinite, crystallized within the ooliths (fig. 1) and in the shrinkage cracks (fig. 2) of the ironstone, and sometimes replaced the ooliths and groundmass. The francolite is present in all of the three oolitic ironstone occurrences described from Yorkshire, and also in the similar rock from Derbyshire in the Geological Survey's collection of iron ores. Details of these localities are given in the ironstones paper. The writer has also found similar francolite in an ironstone nodule from a parting in the Mühlenbach coal seam of south Limburg, Netherlands. Although not recorded, it may be expected to occur in some of the black-band ironstones and other rocks closely associated with coal seams. The following account is based entirely on material from Robin Hood quarry, Thorpe-on-the-Hill, 4 miles south of Leeds.