Potato tubers infected with Heterodera marioni having been received at this Institute, it was decided to attempt to infect a small field plot with this eelworm for experimental purposes. The tubers were planted in the plot but the infection never became established. This was thought to be due to the heavy nature of the soil, since H. marioni has been recorded as a field parasite in Britain (Triffitt, 1931), and it is therefore unlikely that climatic conditions were the cause of its failure to become established in this instance. In order to verify this, and to find out how long this parasite can survive in soil in the absence of a host, the following experiment was carried out.