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An X‐ray diffraction study of CsDNA has been carried out with the use of synchrotron radiation of wavelength λ = 1.2 Å. The geometry corresponds to a cylindrical specimen brought into a cylindrical beam of the same diameter and the absorption factor and the primary‐beam attenuation factor have been calculated as functions of specimen μR for this geometry. The optimum size of a specimen is here about 1.5 times greater than that in the case of a plane‐parallel beam used in International Tables for X‐ray Crystallography [Vol. II, (1967), 2nd ed., p. 295. Birmingham: Kynoch Press]. Experimental and calculated primary‐beam attenuation factors have been compared and the experimental value of the linear absorption coefficient μ of CsDNA has been estimated for various humidities. The agreement between μc = 78.1 and μo = 78.2 ± 3.6 cm−1 (relative humidity 66–75%) indicates that the geometrical model used is suitable to describe polymer fibres in a cylindrical beam of nearly the same size. It is seen from the observed μ values that there is one caesium ion per nucleotide (monomer unit of DNA molecule) in the specimens used.

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