The causes, effects and extent of minnow Phoxinus phoxinus introductions in Norway are reviewed to assess why the introductions have had severe effects, especially where brown trout Salmo trutta is the only fish species present. The natural distribution of minnow in Norway was mainly restricted to low altitude localities in the south-eastern part of the country and in some northern areas. The distribution area expanded considerably throughout the 1900s, especially in mountain areas, due in part to the use of minnows as live bait for angling. Although minnow densities do not seem unusually high in the relatively complex fish communities of its native range, the species can achieve very high population densities when introduced to communities with few fish species, such as in the numerous recently invaded lakes where brown trout was the only fish species present. The dense minnow populations in these lakes appear to have led to reduced recruitment and growth rates in the brown trout, with abundances on average 35% lower in lakes where minnow has been introduced. The success of minnow in harsh habitats demonstrates their phenotypic and ecological plasticity, but also implies that their original distribution in Norway was restricted by early immigration history and not by environmental limitations. This suggests that human-assisted spread of the species could have strong adverse effects in Scandinavia lakes of low fish species richness.