A dynamic model of a diamond mesh cod-end subject to harmonic forcing is developed. The partial differential equations governing the displacements of the cod-end and the tension in the twine are first derived and then analyzed using the harmonic balance method by substituting a harmonic series for the dependent variables and the forcing term. A closed-form solution is derived for the case of rigid-body motion, where there is no deformation of the cod-end geometry, along with the conditions for the forcing under which this motion occurs. A pressure loading, which varies linearly over a portion of the cod-end and varies harmonically with time, is then introduced as a first representation of the loading on the cod-end that results from the pressure and acceleration forces on the catch due to surge motion of the towing vessel. The resulting sets of equations for the static and the first and second harmonic terms are solved numerically in a sequential manner, and the results presented for a number of cases. These results show that, due to the nonlinearity of the system, the oscillatory motion of the cod-end is asymmetric, and that the deformation of the net and the amplitude of oscillation increases as the region over which the forcing is applied increases. The model is the basis for a more complete coupled catch/cod-end model.