Starch composition (amylose content) and the activities of starch-synthesizing enzymes were determined in cultures ofPolytoma uvellaand potatoes kept at various temperatures. In general ADPG-α-glucan glucosyltransferases were much less thermostable in vivo than P- and Q-enzymes. Potato phosphorylase proved to be the most stable enzyme. InPolytomaboth P/Q ratio and amylose content decreased at 30 °C, whereas they remained unchanged in potatoes grown at 30 °C. This confirms the results from earlier experiments which indicated that the P/Q ratio is correlated with amylose content, and is also in agreement with the simultaneous increase in amylose percentage and P/Q ratio observed during the development of starch granules in corn endosperm. The results are discussed in relation to the problem of amylose production in starch granules and the importance of phosphorylase in starch synthesis.