AbstractA field experiment was conducted to clarify the factors responsible for poor seed set in four white clover (Trifolium repensL.) cultivars. Although the mean number of ovules per floret was 5·18–5·29, the average number of seeds per floret was found to be only 1·23‐1·82. A stainclearing technique was used to examine the cytoplasmic state of the embryo sac in intact, unfertilized, mature ovules and it was found that 28–33% of ovules per carpel were sterile. The abortion rate of fertilized ovules and immature seeds (7 days after pollination) ranged between 32 and 34%. Beyond this period, it was observed that only a very small proportion of the total seeds were lost. The observation of high pollen fertility and a high pollen load strongly suggests that poor pollination does not account for the reduced number of seeds per floret. A ‘nutrient competition’ hypothesis was proposed for high ovule sterility and post‐fertilization abortion of d