SUMMARYFive hundred urinary calculi have been examined for direct or indirect evidence of radial striation. Such evidence is found in 42 per cent. of the calculi. Radial striation is limited to areas composed of calcium oxalate monohydrate, uric acid, or calcium acid phosphate dihydrate. It does not occur in areas of mixed composition. Radial striation is always associated with concentric lamination, but the reverse is not the case.Radial striation is shown by X‐ray diffraction techniques to be a consequence of the arrangement of the minute crystals which form the calculus; all the crystals tend to have a particular crystallographic direction radially placed in the calculus. In the case of calculi composed of calcium oxalate monohydrate the particular direction is thebcrystallographic axis; in the case of calculi composed of calcium acid phosphate dihydrate it is theacrystallographic axis.The pathogenesis of radially striated calculi is developed from information on related concretions. The mucinous layer present on calculi is regarded as necessary for the development of this type of calculus: it allows diffusion of ionic groups but prevents disturbances in the process of crystallisation and so permits oriented growth to occur. The mucinous layer is continuously incorporated into the calculus as its “organic matr