This study investigated the effects of temperature, photoperiod and irradiance on the growth rates of two species of filamentous Antarctic macroalgae. Geminocarpus geminatus and Cladophora repens both had low temperature requirements for growth of only -1.5°C, which is the lowest documented to date. Geminocarpus geminatus grew with normal development and morphology at -1.5°C, +1.5°C and +4.0°C. Although G. geminatus was able to survive at +8.0°C, its morphology was not normal. Cladophora repens was more eurythermal in comparison with G. geminatus, growing with normal development and morphology at -1.5°C, +1.5°C, +4.0°C and +8.0°C and with an ability to survive at +12.0°C, although with abnormal morphology. Temperature, photoperiod and irradiance all played important and interactive roles in both growth rates and morphology of G. geminatus and C. repens. Geminocarpus geminatus appeared to have a direct life-history.