It was found that diversity of spidersin Kolyma River delta (68.66º–69.25ºN), located in thesouthern tundra, is about the same as in four other wellstudied local spider faunas located above the PolarCircle in boreal and tundra zones. But three families,Gnaphosidae, Philodromidae and Salticidae, have higherspecies diversity in the Kolyma River delta than inother local faunas of the boreal and tundra zones. Tounderstand reasons for this phenomenon, we comparedzoogeographical aspects of spiders from NortheasternSiberia and Finland and analyzed climate and microclimates indicators in five localities. Most likely the highest species diversity of named families in the tundrazone and even highest diversity among all local faunaslying north of Polar Circle can be explained by a combination of two major factors: history and microclimate. The Kolyma River delta was never covered bythe ice shield like other compared areas; and its highclimate continentality led to a higher heat supply of thesoil surface in the warmest habitats.