The fumigation activity of phosphine (PH₃) and ethyl formate (EF) and their phytotoxicity to 13 imported foliage nursery plant species were evaluated. The lethal concentration and time (LCT₉₉) values of the PH₃ indicated that the susceptibility of the nymphs (3.95 and <0.45 mg·h/liter, respectively) was higher than that of the adults (5.29 and 3.66 mg·h/liter, respectively) of two mealybugs [Pseudococcus longispinus (Targioni-Tozzetti) and P. orchidicolaTakahashi]. The highest concentration reduction rate of PH₃ and EF on the 13 foliage nursery plants in the 12-liter desiccator was 41.5% for Heteropanax fragrans and 71.7% for Schefflera arboricola, respectively, which indicates that PH₃ has a lower sorption rate than EF. The phytotoxicities of PH₃-treated foliage nursery plants did not significantly differ from those of the nontreated plants, but EF caused phytotoxicity in 11 foliage nursery plants a week after treatment. When the exposure time of PH₃ increased to 24 h, the adults and nymphs of both mealybug species showed 100% mortality in the 0.5 m³ fumigation chamber. In the 10 m³ fumigation container used in the field, there was 100% mortality of both mealybugs after treatment with 2 g/m³ PH₃ for 24 h at 16°C. These results indicate that EF is not a suitable mealybug fumigant due to its high sorption and phytotoxicity to foliage nursery plants, despite fumigation activity against the two species. However, PH₃ seems to be suitable for mealybug fumigation in foliage nursery plants and can be used as a substitute for methyl bromide.