The Antarctic and Arctic fish faunas differ in age, endemism, taxonomic diversity, zoogeographic distinctiveness and physiological plasticity. The Antarctic fauna includes 274 species representing 49 families; the Arctic fauna 416 species in 96 families. Using 5% as a figure for recognition of dominant faunal elements, five groups account for about 74% of the Antarctic fauna (notothenioids, myctophids, liparids, zoarcids and gadiforms). With 35% of the species, notothenioids prevail in this assemblage. Together with the liparids, they are the only known example of an adaptive radiation of fish in a marine habitat. In the Arctic, six dominant groups comprise 58% of the fauna (zoarcoids, gadiforms, cottids, salmonids, pleuronectiforms and chondrichthyans). In the coldest water at the highest latitudes (> 65°S or N), only zoarcids and liparids are shared by both faunas. In the Antarctic species, endemism is 88% for the benthic fauna of the shelf and upper slope (174 species) and rises to 97% when only notothenioids are considered. Comparable figures in the Arctic are 20-25% for marine fish and 2% or less for freshwater and anadromous species. The high endemicity in the Antarctic reflects the isolation and age of the ecosystem: 22 million years versus only 0.7-2.0 million years for the Arctic. The Arctic fauna consists of boreal marine and freshwater fish which are generally more eurythermal and euryhaline than their Antarctic counterparts. The North Atlantic and North Pacific character of the marine fauna reflects the continuity of shelf areas between the Arctic and boreal regions. There are examples of convergent evolutionary trends in Antarctic and Arctic species that share habitat similarities. These trends are evident at several levels of diversity: intraspecific (genetic), specific (organismal) and ecological.