1 Experiments carried out during 1933-8 with the object of investigating the control of skin spot (Oospora pustulans Owen & Wakef.) by seed-tuber disinfection are described. 2 Compounds of mercury were effective when used in solution as steeps. Formalin and a proprietary dusting compound proved to be non-effective. Proprietary organo-mercury compounds gave the most promising results, and were satisfactory when used as “instantaneous” dips (0·5-1·0 min.); mercuric chloride solution was efficient but the 90 min. dipping period employed made the method too tedious. 3 The time at which the treatment is carried out is of great importance in determining the measure of control obtained. Disinfection of the seed at the time of digging was far more effective than immediately before planting. If too long an interval (more than 8 weeks) is allowed to elapse between digging and disinfection, the value of the treatment is substantially reduced. 4 A study of the time of digging the crop upon the incidence of the disease, made during one year only, indicated that for that year early digging did not reduce the amount of the disease which developed during subsequent storage. Disinfection carried out at each of the five diggings gave satisfactory control in each case. 5 The method of treatment recommended is the same as that previously suggested for the control of common scab (Actinomyces scabies (Thaxt.) Güss.) and blight (Phytophthora infestans (Mont.) de Bary). Some evidence was obtained of its also being effective against storage diseases. It would, therefore, appear that disinfection of seed tubers at the time of digging may prove effective against a group of tuber diseases many of which are transmissible to the resultant crop.