Summary1An experiment with three varieties of potatoes (Great Scot, Kerr's Pink and King Edward) was carried out during the period 1922–24 in order to test the physiological theory of potato deterioration by determining whether the cropping power was influenced by early lifting or affected by shading the growing plants and so protecting them from overheating. The main results of this experiment were as follows:aIn the absence or very limited presence of virus disease, neither early lifting nor shading produced any significant effect on crop yield. All the types of seed (one‐year grown, two‐year grown, early‐lifted, etc.) gave yields which were not significantly different from that of fresh Scotch seed.bWhen considerable infestation with virus disease was shown, all the types of English‐grown seed showed a significant falling off in yield as compared with Scotch seed. In such cases the falling off in yield was not so great with seed which had been lifted at a very early date. Thus, with virus disease present, an advantage may be gained by early lifting of the seed.cIn one case, evidence was obtained that very immature seed is intrinsically inferior in cropping power to seed lifted at the normal time, i.e. apart from the effects due to the unequal distribution of virus disease.dIn two cases the greater cropping power of one‐year grown English seed as compared with fresh Scotch seed was demonstrated. This difference is probably to be explained on the ground of the unequal sprouting at the time of planting.eA general correlation was observable between the amount of virus disease present in the various plots and the falling off in cropping vigour.fNo effect of a purely physiological nature arising from early lifting or from shading was indicated. Though such an effect could not be disproved, it was clear that it was of minor account and quite subsidiary to the effects of virus disease.2An experiment was carried out over the period 1925–8 to determine whether, by a process of rogueing, the cropping vigour of a deteriorated stock could be restored. The weight tests carried out in 1928 showed that certain English‐grown stocks, some of which were in their seventh year from Scotland, yielded as heavily as Scotch seed.3The experiments were carried out in such a way that the statistical significance of the data obtained could be assessed. In thirty‐three such experiments, where the yield of English‐grown types of seed was compared with that of Scotch seed, the percentage drop in cropping power which was significant varied from 7.0 to 20.0. Over the whole series, the average figure was 11.8 per cent., corresponding to a falling off in yield of about 1.2 tons per acre.In conclusion we wish to record our thanks, for facilities offered for this work, to the Director of the Rothamsted Experimental Station and to the Principal of the South‐Eastern Agricultural College, Wye, and the Principal of the Harper‐Adams Agricultural