From three field isolates ofPhytophthora infestans sub-isolates were set op on different potato varieties. Their growth rates were assayed on tuber tissue of several potato varieties. Neither in respect of the variation between the original field isolates nor variation arising during the experiment was there any evidence of adaptation by the fungus to a particular host variety. It is concluded that, if such adaptation is possible, it must be small in comparison with the overall variation in growth. The relevance of this finding to the problem of field resistance is discussed. and it is suggested that it accords with the general experience of varieties in cultivation.