During the course of recent hydrographic and oceanographical investigations in the Pacific, H.M.S. Challenger made considerable use of the extensive Loran coverage now provided by the United States. This marine radio position-fixing aid was particularly useful in the west Pacific in the area between Japan and New Guinea where the coverage of ground-waves is considerable. In this article an attempt is made to describe some of the work for which Loran was used, the principles on which it was used, and the accuracy expected and errors obtained. No attempt has been made to describe the operation of the system, which has been adequately covered in this Journal and in numerous other publications.