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This investigation was conducted as part of a program of research designed 1) to develop several methodologies for the evaluation of diver communication systems and 2) to carry out these evaluations on available units. The major focus of this report is on a diver-to-diver procedure and data resulting from the evaluation of seven diver communication systems, viz: a) hard-lineAquaphone; b) acoustic-Bendix and Yack-Yack; and c) amplitude modulated-PQC-1a, PQC-2, Aquasonics 811, and Aquasonics U-42. All systems were used with a Nautilus muzzle and a double-hose regulator. Talkers were five divers experienced in such tasks; listeners were six to eight divers familiar with the talkers' speech; stimulus materials were the Clarke 50-word multiple choice lists. The following results were noted: 1) no single approach to diver communication (modulated, acoustic, hard-line) completely dominated the results; 2) the use of optimum (currently available, that is) muzzle-regulator combinations, a closed set of speech materials, and procedures where talkers' speech was familiar to the listeners, all operated to improve the intelligibility of communication; 3) the performance of the military systems was poorer than that of the commercial ones; and 4) these systems-even when used optimally-still do not provide adequate communication for divers.