Experiments at two localities in the interior of British Columbia in 1952 and 1953 showed that a seed treatment of dieldrin at 0.5 oz. per pound of seed gave as good control of the onion maggot as any other treatment, was not phytotoxic, and gave the highest yield of marketable onions each year. Lindane, 25 per cent wettable powder, applied three times at 10-day intervals to the soil surface at 1 lb. of toxicant per acre per application gave consistently good control and high yields, but was more expensive in both labour and materials. Calomel at 1 lb. per pound of seed gave satisfactory control in a light infestation but cost twenty times as much as dieldrin. DDT at 8 oz. per pound of seed gave effective control but the bulk of insecticide on the seed caused jamming of the seeder. When the amount of DDT was reduced the degree of damage increased. Lindane as a seed treatment at 1 oz. per pound of seed was extremely phytotoxic. The same amount of aldrin applied in a similar manner was phytotoxic but to a lesser degree.