Some preliminary experiments on the effects of potato root excretions in stimulating the larvæ of H. schachtii to hatch and emerge from the cyst, and the neutralisation of this stimulating factor by the root excretions of mustard seedlings, have already been described. Since chemical treatment of infected soil as a means of controlling the eelworm has been the subject of investigation in Europe for the past fifty years without yielding any results of practical value, investigations into the reactions of the paiasites to the root excretions of plants were undertaken in the hope that some result might be attained which would point to a solution of the eelworm problem on biological lines. The high degree of specialisation shown by the British potato-strain rendered it peculiary suitable for investigations of this character.