I studied the occurrence of two sympatric morphs of arctic charr, Salvelinus alpinus (L.) in Vatnshlidarvatn, a small shallow lake in NW Iceland. The arctic charr were subjectively distinguished by colour and appearance as brown morph or silver morph, and measured for morphological and life history characters. The study revealed the presence of two growth forms represented by the two morphs that differed in age and size at sexual maturation, reproductive investment, and time and place of spawning. The morphs differed significantly in gill raker number and morphometric characters related to manoeuvrability and cruising ability. Morphological segregation was established early in life and is most readily explained as developmental heterochrony. Both morphs were benthivorous, but could be segregated as diet specialist and generalist, with diet segregation being important only when food (especially the benthic crustacean Eurycercus spp.) was scarce, The occurrence of one abundant food resource, and lack of interspecific competition (no other fish species are present) may explain the different feeding strategies. The presence of "empty" niche should induce variability and divergence in morphology and life history to occupy available niche space. / Graduation date: 1997