In most terrestrial ecosystems of boreal and northern climate zones degradation of organic matter with methane production occurs at low temperature. Two psychrophilic methanogenic archaea and four acetogenic bacteria were described until now. Recently, we isolated 12 new strains of methanogenic archaea and 3 strains of acetogenic bacteria from different natural and man-made cold environments including tundra permafrost wetland, sediments of deep lakes, silt of sludge disposal pond, pig and cattle manure digested at 6°C, and an anaerobic EGSB-reactor operated at 9°C. The temperature characteristics of microorganisms isolated from cold environments are discussed. All isolates are able to grow below 10°C, most of them grow at such low temperature as 1°C. The upper temperature limit for most growing at low temperature acetogens is 30°C, and the temperature optima is 20°C and below. Most isolated methanogens have temperature optima around 25°C, and upper temperature limits at 30-40°C. Whether microorganisms able to grow at low temperature are classified as mesophiles, psychrophiles, or psychrotrophs (psychrotolerants) is an issue of this article. We propose to modify the basic temperature definition of anaerobic microorganisms growing at low temperature.