An individual-based model that simulates fish selection processes in diamond mesh cod-ends of towed fishing gears is outlined. The model is implemented in a computer program called PRESEMO. A typical simulation can be carried out within a few minutes on a personal computer. Up to four different populations of fish entering the cod-end during a tow can be accounted for. Each fish is assigned a weight, girth, width and height according to its length, and is assumed to have an elliptical cross-section. Fish are allocated a period of travel time down the cod-end, a period for swimming in the cod-end without being exhausted, a period between escape attempts and a packing density for those swimming ahead of the catch. An escape attempt is deemed successful if a fish can pass through the mesh opening at the position in the cod-end where the escape attempt takes place. The mesh opening value is obtained from information on the shape of the cod-end, which depends on the catch weight. The cod-end shape is updated dynamically as the catch builds up during the tow. During a simulation the selection process is continually visualized, that is, the entry, movement and escape attempts of individual fish are shown as well as the changes in the cod-end geometry.At the end of a simulation, a logistic function is automatically fitted to the selection data to obtain estimates of the 50% retention length and the selection range.