The synthesis and accumulation of tannins on tomato seedlings are regulated by environmental factors. The variation in the content of tannins was sufficiently important to bring about the occurrence of significant differences in the numbers of glasshouse whitefly on the seedlings. During a 2-week experiment, the treatments included mechanical wounding (20 prickings per cm2), spraying with kinetin solutions of 10−4 mol/dm3, plant growth regulators, and the atmosphere enrichment to 680 μmol CO2/mol air, the content of tannins being increased by c. 40, 70, 10–45 and 25% above the values obtained in the control. These results were correlated with a decrease in the numbers of insects occurring on the seedlings by c. 35, 45, 8–29 and 18%, respectively. Contrary to the above results the spraying with solutions of abscisic acid, gibberellic acid, and the incubation of plants in an atmosphere containing 170 μmol CO2/mol air, reduced the content of tannins by c. 69, 22 and 25%, respectively. This was reflected in the respective increases by c. 70, 40 and 35% in the numbers of insects occurring on the seedlings. The obtained results suggest that tannins seem to have a dosage-dependent effect on glasshouse whitefly. Decreasing the host plant quality by increasing tannin content may act as an important selective agent limiting the losses brought about by glasshouse whitefly in tomato cultivation.