ABSTRACT: Sea ice provides a habitat for a diverse community of microorganisms, which comprise a substantial portion of primary production in ice-covered seas. Organisms immured in sea ice have to withstand strong changes in temperature and salinity. We report on the growth rate response to salinity and temperature of the chlorophyte Chlamydomonas sp. ARC, isolated from land-fast sea ice in the Chukchi Sea, Alaska. We found it to be a euryhaline psychrophile capable of growth at temperatures as low as –5°C and at salinities from 2.5 to 100‰. The maximum growth rate of 0.41 d^–1 (±0.027) was found at 5°C and a salinity of 30‰. The salinity growth range of this organism indicates that it is well adapted to the variable salinity environment associated with brine channels in sea ice, as well as the hypotonic conditions associated with melting ice. Based upon morphology and molecular phylogenetic reconstructions using the 18S ribosomal ribonucleic acid (rRNA) gene and the ribulose 1,5-bisphosphate carboxylase/oxygenase large subunit (rbcL) gene, this Arctic Chlamydomonas falls into a distinct clade containing other as yet unassigned psychrophilic Chlamydomonas strains isolated from Arctic and Antarctic environments, pointing to a bi-polar distribution of this clade. It is also very closely related to the brackish water mesophile Chlamydomonas kuwadae Gerloff, and is capable of growth above the psychrophilic range in low-salinity medium, indicating that it may represent an intermediate between mesophilic and psychrophilic lifestyles.