A sound-propagation model based on normal mode theory is described. The model is designed to give a realistic treatment of the ocean environment, including arbitrary sound-speed profiles in both water column and bottom, compressional and shear wave attenuation, scattering at rough boundaries, and range dependence. Furthermore, the model has a flexible inputoutput structure that facilitates model handling and provides users with a wide choice of computational output options, ranging from plots of sound-speed profiles and individual mode functions to contoured transmission loss versus depth and range or versus frequency and range. The computer code is written in FORTRAN V with a few routines in NUALGOL. The version documented here runs on a UNIVAC 1106.