The three main groups of cultivated potatoes were studied (diploids (2n ----24), Andigena (2n = 48) and Tuberosum (2n ~ 48)). Seed weights were measured as mg per 100 seeds. In the tetraploids, but not in the diploids, there is a regression of seed weight on seed number per berry with b --~ -7 mg per 100 per 100 seed, implying nutritional competition between seeds. This effect accounts for very little variation between samples however. Tetraploid seeds are about 30 % larger than diploid seeds but there is much overlap between the two groups; genetic control of seed number appears to be predominantly bi-parental, of seed size predominantly maternal, but there are exceptions. Tuberosum seed is slightly larger than Andigena seed and the difference is attributable to the decline in fertility that occurred during the evolution of the former Group. There is no evidence of increase of seed size correlated with selection for plant vigour; experiments with sieved seed of different size grades showed that genetic differences independent of seed size overwhelmingly controlled plant vigour but that there was a transient maternal effect of seed size apparent in the young seedlings and disappearing later in life. General conclusions are: that seed size in the cultivated potatoes is determined by ploidy, by genetic factors (mostly maternal) and by maternal nutritional effects; and that it offers no correlations with plant characters that might be useful to the plant breeder.