Cyanobacteria are often the dominant phototrophs in high-latitude lakes and streams where they must experience continuously low temperature and extreme variations in daylength. The present study examined the interaction between these two variables for the growth and physiology of an Arctic isolate of the mat-forming species Schizothrix calcicola (Agardh) Gomont. Growth rates (μ), photosynthesis (P), respiration (R), pigment composition and in vivo absorption characteristics were measured under 15 combinations of daylengths (8[ratio ]16, 12[ratio ]12, 16[ratio ]8, 20[ratio ]4 and 24[ratio ]0 L/D) and temperatures (5, 15 and 25 °C). μ increased with increasing temperature (μmax at 5 °C = 0·12 d−1, μmax at 25 °C = 0·28 d−1) and a similar trend was observed in photosynthetic capacity (average PBmax at 5 °C = 4·42 mg C (mg Chl a)−1 h−1, average PBmax at 25 °C = 5·74 mg C (mg Chl a)−1 h−1), pigment content and absorbance. Daylength had a positive effect on μ and pigment absorption, but not pigment content. The shape of the μ–daylength curve varied with temperature: μ was a linear function of daylength at 5 °C, but at 15 and 25 °C the relationship resembled a rectangular hyperbola and μ saturated at 16[ratio ]8 and 12[ratio ]12 L/D, respectively. The non-linear relationship between μ and daylength at high temperature was related to a reduction in net photosynthesis under extended daylength; at 25 °C, net P under 24[ratio ]0 L/D was 0·82±0·37 mg O2 (mg Chl a)-1 h−1, while under 8[ratio ]16 L/D, it was 6·54±0·69 mg O2 (mg Chl a)−1 h−1. The constant increase in growth with increasing daylength at low temperature may reflect an adaptive tolerance to the combination of cold temperature and continuous daylight during the Arctic summer.