Although it is generally believed that cyanobacteria have high temperature optima for growth (> 20" C), mat-foming cyanobactm'a are dominant in many types of lakes, streams, and ponds in the Arctic and Antarctic. We studied the effect of temperature on growth (p) and relative pigment composition of 27 isolates of cyanobactm'a (matforming Oscillatoriaceae) from the Arctic, subarctic, and Antarctic to investigate whether they are a) adapted to the low temperature (i.e. psychrophilic) or b) tolerant of the low temperature of the polar regzons (i.e. psychrotrophic). We also derived a parabolic function that describes both the rise and the decline of cyanobactm'al growth rates with increasing temperature. The cyanobacteria were cultured at seuen different temperatures (5"-35" C at 5" C interuals), with continuous illumination of 225 pmol photons.m-2.s-1. The parabolic function jits the p-temperature data with 90 % conjidence for 75 % of the isolates. Among the 2 7 isolates of cyanobactm'a studied, the temperature optima (T,J for growth ranged from 15" to 35" C, with an average of 19.9" C. These results imply that most polar cyanobacteria are psychrotrophs, not psychrophiles. The cyanobactm'a grew over a wide temperature range (typically 20" C) but growth rates were low men at T& (average pmax of 0.23 2 0.069 d-I ) . extreme^ slow growth rates at low kmperature and the high temperature f i optimal growth imply that the cyanobact,ria are not adapted genetically to cold temperatures, which characterize their ambient environment. Other competitive advantages such as tolerance to desiccation, freeze-thaw cycles, and h g h t , continuous solar radiation may contribute to their dominance in polar aquatic ecosystems.