We investigated the relationship between seed size and seed survival in the soil in 67 species from arid Australia. There was a very weak, marginally significant positive relationship between the viability of fresh seeds and diaspore mass. However, by the time seeds had been buried in the soil for 1 year in nylon mesh bags, there was a highly significant positive relationship between diaspore mass and diaspore viability. Over the range of seed masses observed, a tenfold increase in diaspore mass was associated with a threefold increase in the odds of surviving 1 year of burial in the soil. Thus, large-seeded species were favoured over small-seeded species during this important selective process. However, the magnitude of this advantage was small compared with the advantage experienced by small-seeded species during seed production. We also investigated aspects of diaspore morphology in relation to viability retention during burial. We found no relationship between seed survival and the amount of protective tissue per unit diaspore surface area. Diaspore mass was a better predictor of survival than was diaspore surface area.