A large fluid lens with accompanying reservoir and pump arrangement is described, with emphasis on its underwater acoustic‐imaging application. Pertinent properties are cited of the neutrally buoyant lens fluid, the elastomeric containment membranes, and the composite lens. The liquid‐filled lens potentially provides better over‐all acoustic transmissivity than does a comparable solid lens and has an equivalent acoustic refractive index. Focusing, achieved without physical motion of the lens proper, occurs in a manner analogous to the operation of the human eye. Introduction or removal of fluid changes the curvature of the lens‐membrane surfaces. Theoretical consideration reveals the stretched‐membrane surface configuration to be approximately spherical. Thus, standard spherical lens equations are combined with appropriate mensuration formulas to obtain lens calibration equations. A hydroacoustic image taken with the liquid lens is displayed. This image is compared with that formed under identical circumstances by means of an equivalent Lucite lens. Beamwidth and spherical aberration of the fluid lens are discussed. A field stop for use with the liquid lens and a technique for immunizing the lens to water‐wave action are noted.