The object of the investigations reported in this paper was to determine the actual values of the friction coefficients for the flow of air, oil, and water through pipes of annular cross-section. The apparatus and method of testing are described and the data and results of the tests are given in graphic and tabular form. It appears that the coefficient of friction corresponding to any given turbulence factor for both turbulent and viscous flow has a value for pipes of annular cross-section slightly higher than for pipes of circular cross-section when determined by the equivalent-diameter formula.