A factorial experiment involving the presence and absence of three factors: steam sterilization (S), infection with some three million eggs and larvae of Heterodera rostochiensis per pot (H), and injection of 10 ml. of D-D per pot (D), the factors applied in that order, was carried out to ascertain the effects on the growth of 40 potato plants in pots containing 15 Kg. of soil.In the event, the D-D was lethal to the great, majority of eelworms, so that treatments labelled (hd) and (hsd) were effectively reduced to (d) and (sd). This fact greatly complicated the analysis, but nevertheless enabled certain conclusions to be drawn. The use of this factorial design may be criticised in the face of this antagonistic action between, two of the factors. Nevertheless, the effects of H and D acting together on the plants was a subject of interest; the real fault in design lay in using too high a concentration of D-D, or in not using a series of concentrations. The fact that the concentration used was too high emerged from the experiment and was not known beforehand.