Ship motions are recognized by fishermen as a cause of high musculo-skeletal load. In the present study, the motions of a Swedish trawler at sea were registered over three degrees of freedom. Simultaneously, the working postures of a fisherman on board were registered in the sagittal plane during five different working situations: (1) standing erect in still conditions; (2) holding a load of 21 kg in still conditions; (3) standing erect during motion of the ship; (4) standing during motion of the ship, holding a load of 21 kg; and (5) repeatedly lifting and lowering a 21 kg load during motion of the ship. All registrations were sampled using a computer where data were processed in a two-dimensional, dynamic biomechanics model, developed for this particular purpose. Vertical and horizontal forces as well as moments were calculated for seven major joint systems of the body, as was compression at the L4/L5 vertebral level. In situation (3), ship motions were mainly counteracted by motions in the lower extremity and lumbar back, thus inducing increased strain in these parts of the body; other parts of the body were little affected. Holding a load considerably increased the load on most joints. Lifting the load further increased the musculo-skeletal strain and also increased the range of moments in each joint as well as the range of lumbar compression. Moment at the C7 vertebral level was relatively unaffected by ship motions and by handling external loads.