In eutrophic Lake Alpnach, several forms of whitefish (Coregonus spp.) are found which show different growth rates. In recent years, immature whitefish of the fastgrowing form (local name: ‘Felchen’, here called ‘Grossfelchen’) have been caught in the nets used for fishing the slowgrowing form (local name: ‘Albeli’, here called ‘Kleinfelchen’). The aim of this study was to identify the different forms of coregonids and to determine their growth rate, age of first maturation, and relative frequency in the catch with gill nets of different mesh sizes. While the separation of the different forms on the basis of meristic characters proved unsatisfactory, the growth during the first year of life was found best suited for this purpose. For the whitefish fishery in Lake Alpnach, the fastgrowing form is regarded as the most important one. Due to eutrophication, the whitefish now show a considerably faster growth than about 40 years ago. In addition, they mature at a younger age than before. The consequences of these findings for the management of the whitefish stocks in Lake Alpnach are discussed.