At present the longitudinal strength of cargo ships is specified by the construction rules of classification societies, the strength standard of the International Load Line Convention, etc., but the basis on which these rules have been developed is not always clear. The author dealt with the longitudinal strength required for cargo ships from two different view points, viz. the longitudinal strength based on the ultimate strength and that based on the fatigue strength, and as to each of them developed formulae giving the section modulus amidships required for cargo ships, introducing the corrosion margin of 3 mm for oil tankers and 2 mm for general cargo ships, and using the formulae for the longitudinal bending moment proposed by the author in his previous paper as a result of statistical analysis and a constant allowable stress common to all types of ships. The comparison between the section moduli obtained from the above formulae and those required by existing rules indicates that the average degree of safety on the longitudinal strength of cargo ships when given by the above formulae is in substantial agreement with that given by existing rules, but the scatter of the degree of safety in individual ships according to ship's type, form, etc. will be reduced to a considerable extent. It is hoped that this paper would contribute to the progress of the rules for the longitudinal strength of ships.