The influence of the biological insecticide Botanigard (Beauveria bassiana) on different developmental stages of the greenhouse whitefly (Trialeurodes vaporariorum) has been tested and compared with the influence of Preferal (Paecilomyces fumosoroseus), also a biological product. Six experiments were set up to test the two products on eggs, which were four and seven days old, on larvae of the first, second/third and fourth stage and to test the effect on egg-deposition. These experiments were all conducted on cucumber. Egg-deposition was limited to a small area on the leaf by using leaf cages. To evaluate these tests the number of eggs or larvae developed to the next stage has been counted and compared to the total amount of eggs or larvae on the leaves. The results revealed that Botanigard has an effect on the larval stages. The first larval stage is most sensitive; the next stages have a decreasing sensitivity. There was no influence on the hatching of the eggs, but a treatment short before the hatching could have a residual effect on the new nymphs. When the treatment with Botanigard is performed shortly before a moult or a fungicide treatment, the efficacy of the product decreases significantly. The influence of Preferal on the greenhouse whitefly is, under the same circumstances, less obvious.