There is some evidence that microbes inhabiting permanently frozen environments are capable of cell maintenance and growth under in situ conditions. In contrast, Arctic tundra surface soils that are only frozen during the winter have exhibited CO2 respiration under frozen conditions. This could result from maintenance metabolism fuelled by recycling cell material, but does necessarily indicate growth, which requires the synthesis of new cell material. We used 13C-glucose to trace C into membrane lipids and 5-bromo-3-deoxyuridine to monitor DNA synthesis in a laboratory microcosm study on Arctic tundra soils. Organisms were not equally active: fungi incorporated more C than any other group. Although some bacteria were growing, Gram(+) bacteria were almost completely inactive. Shrub tundra microbes were more active in early winter than tussock microbes, incorporating more C and synthesizing more DNA. In late winter, the C incorporation pattern reversed, although DNA production was similar. We demonstrated for the first time that microbes in frozen tundra soils synthesized new cell membranes and DNA, processes fundamentally associated with growth. That microbes can grow during Arctic winters may have important implications for C cycle modelling under a changing climate.