A 3-yr study was conducted on three synthetic Brassica rapa L. cultivars to determine the effects of seed size and seed weight on seedling establishment, seedling growth and susceptibility to feeding damage by flea beetles, Phyllotreta spp. (Coleoptera: Chrysomelidae). Seed lots of AC Boreal, Fairview and Hysyn 110 were sieved to obtain small, medium and large seeds (1.4–1.6, 1.6–1.8 and 1.8–2.0 mm, respectively). In the laboratory, seedlings grown from large seeds had the largest cotyledons, highest shoot dry weight and highest biomass. Shoot weights increased as seed size increased. Sized seeds of the three cultivars were grown in the field without insecticides in 1998–2000. Seedlings of small seeds had the highest flea beetle damage and poorest seedling establishment. Shoot dry weight and biomass 14–35 d after planting increased as seed size and seed weight increased. Compared with small seeds, large seeds improved shoot dry weight, biomass and seed yield by 13–43, 25–57 and 12%, respectively. Results indicated that seedlings of medium and large seeds are more vigorous and tolerant to flea beetle damage than seedlings of small seeds. Tolerance was due to a higher initial seedling weight rather than higher relative growth rate. Shoot dry weights, biomass and yield of the three cultivars were more strongly correlated with 1000-seed weight than with seed diameter. Key words: Canola, flea beetles, seed size, seedling vigour, tolerance, seed weight