Previous investigations (e.g. Bates, 1935; Singh, 1952; Bremner & Taha, 1966; Sharpe & Dent, 1968) established that increase in the size of potato seed planted and decrease in inter-sett spaoing increased the number of stems and of tubers, resulting in yield advantage generally, but reduced tuber size in the produce. The use of large seed and of close spacing are, therefore, generally preferred for growing a seed crop. However, their use involves higher seed rates, especially when the two are combined. At high seed rates the improvement in yield with increased seed rate may not be commensurate with the extra investment in seed, beoause of the associated increase in inter-sprout and inter-tuber competition. The severity of the competition may be intensified when some factor, for example water, limits tuber growth (Singh, 1952). As the number of stems is not a simple multiple of seed weight, because of the phenomenon of ‘apical dominance’ in potato tubers, it may be desirable to plant small seed or ‘chats’ closely, without increasing seed rate unduly, to optimize stem density without sacrificing net yield (total yield –seed planted). To investigate this, a seed size and spacing experiment was conducted under irrigated conditions in the autumn crop season.