Knowledge of the size-selectivity of commercial fishing gear is a prominent topic for fisheries management, marine ecology and protecting fish populations. The trammel net selectivity of most freshwater fish is poorly known. A total of 1,029 specimens were caught by the trammel nets in the Sakarya River, Turkey. In this study, experimental trammel nets using three monofilament (72, 80, 88 mm), five multifilament (64, 72, 80, 88, 96 mm) mesh sizes for the inner panel and one multifilament mesh size (600 mm) for the outer panel were constructed. The SELECT method was used to obtain selectivity for comparing different trammel net mesh sizes. Normal scale model for monofilament nets and log-normal model for multifilament nets gave the best fit. The modal lengths of Capoeta baliki in 72, 80, 88 mm monofilament and 64, 72, 80, 88, 96 mm multifilament mesh sizes were calculated as 30.38, 33.00, 37.14 cm and 26.52, 29.83, 33.15, 36.46, 39.78 cm, respectively. The present study was an attempt to gain a better understanding of net selectivity parameters, especially for C. baliki intensively caught in the study area. For the first time in this area, these parameters were calculated for C. baliki.