An accurate analytic expression of the surface charge density/surface potential relationship for an infinitely long cylindrical colloidal particle in a solution of general electrolytes is derived from an approximate solution to the nonlinear cylindrical Poisson-Boltzmann equation. The mathematical procedure is based on a method developed previously by Ohshima, Healy, and White for the case of a sphere (J. Colloid Interface Sci. 90, 17 (1982)). Comparison is made with exact numerical results. Accurate expressions for the potential distribution around a cylinder and the effective surface potential of a cylinder are also derived. Finally, expressions for the double-layer interaction energy and force between two cylinders at large separations are derived on the basis of the method of Brenner and Parsegian (Biophys. J. 14, 327 (1974)).