This investigation was carried out to confirm the effectiveness of the introduced parasite, Encarsia formosa Gahan, against the greenhouse whitefly, Trialeurodes vaporariorum (Westwood), on short, cold-grown crop in a plastic house. Three kinds of plots were prepared to evaluate the effectiveness of the parasite: (1) release of Encarsia formosa; (2) release of Encarsia formosa and the domestic parasite, Encarsia sp.; (3) the untreated plot. Twenty tomato plants were set out in each of plots on March 23, 1978. Ten individuals of each of newly emerged female and male whiteflies were released on leaves of each of plants on April 11. Parasites were released on May 4 in the form of mummies on each of plants. The number of mummies released per plant of Encarsia formosa and Encarsia sp. was five and six, respectively. The survival rate of the whitefly in the first generation in the plot with Encarsia formosa, with Encarsia formosa and Encarsia sp. and without parasites was 35. 5, 27. 0 and 90. 7 %, respectively. Comparison of the number of adult whiteflies between the plot with Encarsia formosa and the plot without parasites revealed that the number of the former was significantly less than that of the later. The percentage of fruits contaminated with sooty mould in the plot with Encarsia formosa was 5. 7 %, while that in the plot without parasites was 74. 6 %. On the other hand, there was no significant difference in the number of adult whiteflies between the plot with Encarsia formosa and the plot with Encarsia formosa and Encarsia sp. The percentage of fruits affected in the plot with Encarsia formosa and Encarsia sp. was 12.2%. When female adults of Encarsia formosa were as abundant as those of Encarsia sp. the number of mummies produced by Encarsia formosa might have been limited by interspecific competition. However, the population of Encarsia formosa increased more rapidly than that of Encarsia sp.