This guide documents the use and background of a program written for MSDOS Computers, in QUICKBASIC 4.5, for straightforward construction and parametrization of steady-state trophic models of aquatic ecosystems. The ECOPATH II program is structured around a system of linear equations as suggested by JJ. Polovina and coworkers, and also incorporates routines from the computation of various network flow indices derived from a theory presented by R.E. Ulanowicz.