SummaryWhen two different soils were incubated after the addition of [14C] glucose in the dark at winter temperatures or at 5°C in the laboratory and then hydrolysed, radioactivity was detected in all seven common soil sugars except arabinose. In contrast, in incubations at 20°C, little radioactivity was found in the xylose. Examination of the microflora showed that the number of viable bacteria was one‐tenth at the lower temperatures, whereas the numbers of yeasts, fungi, and actinomycetes were unaffected. Analysis of cultures of representative microbial isolates showed that none of the fungi or actinomycetes and only 3 per cent of the bacteria synthesized xylose, compared with 85 per cent of the yeasts.It is concluded that when these soils are incubated at low temperatures xylose is synthesized principally by yea