The interactive effects of pure emulsifiable formulations (10(10) conidia.ml-1) of Beauveria bassiana and Paecilomyces fumosoroseus conidia and 1% and 3% of imidacloprid 10% WP were tested for controlling greenhouse whitefly, Trialeurodes vaporariorum, on lettuce grown in four polyethylene film-covered greenhouses (5 m x 100 m) in Kunming, Yunnan. Six fungal treatments, two imidacloprid ones, and one water-spray control were included in the experiment, with each being replicated in three plots (7 m x 5 m.plot-1). For each of the treatments, 1000-fold aqueous dilution (10(7) conidia.ml-1) was sprayed twice at a 15-d interval, with the first spray given on day 11 after lettuce transplantation. Counts of live and dead whiteflies from each treatment were made on the initial day, and then, once every five days. During a 30-d period of observation from August 20, the two applications of the fungal formulations alone or those containing low rates of imidacloprid 10% WP effectively protected the lettuce from damage by T. vaporariorum, and generated relative efficacies. Whitefly density decreased > 95%, which was significantly higher than that attributed to the low application rates of imidacloprid 10% WP. Based on the estimations of relative efficacy and percent density decrease, P. fumosoroseus treatments controlled the whiteflies better than B. bassiana ones, and the effects of both fungi on the pest population were apparently enhanced with the quantity of imidacloprid added. The differences among the fungal treatments in the two estimations were larger during the first 10 days, but decreased thereafter, becoming insignificant on day 10 after the second spray. The results indicated that the fungal formulations tested in this study are of great potential for practical use in whitefly control, and their efficacy could be enhanced with low rates of imidacloprid.