Laboratory fumigations with sulfur dioxide (SO₂) were conducted under ultralow oxygen (ULO) and normal atmosphere to determine their effects on mortality of grape mealybug, Pseudococcus maritimus (Ehrhorn), and vine mealybug, Planococcus ficus Signoret, and postharvest quality of table grapes. Three- and 4-d fumigation treatments with 100 ppm SO₂ under 30 ppm ULO condition and normal atmosphere resulted in 100% mortality of eggs and nymphs/adults of vine mealybug. The 3- and 4-d SO₂ fumigations under the ULO condition and the normal atmosphere were also effective against grape mealybug with egg mortalities of 85.8% and 98.8% and nymph/adult mortalities of 99.0% and 100%, respectively. There was no significant difference between SO₂ fumigations under ULO and the normal atmosphere in insect mortality. One-day fumigation treatments with 400 and 500 ppm SO₂ achieved 89.8% and 95.8% mortality of grape mealybug eggs, respectively, and achieved 100% mortality of grape mealybug nymphs and adults. None of the treatments used in this study adversely affected quality of the six table grape cultivars used in the studies. SO₂ fumigation under either ULO or normal atmosphere is potentially a useful means for postharvest control of mealybugs on harvested table grapes.