Abstract: The aim of the study was to determine the long-term effects of the fungicide tebuconazole on mortality and fecundity of the parasitic wasp, Aphidius rhopalosiphi and mortality of its host, the bird cherry-oat aphid, Rhopalosiphum padi L. under laboratory conditions. The period after exposure before the insects can recolonize the plants was also determined. The adult wasps and aphids were exposed on barley plants (Hordeum sp. L.) treated with the fungicide. Tebuconazole was tested using a representative commercial formulation. The test item solution was applied to barley twice, at a 14-day interval, using a chamber sprayer. It was tested at a single rate of application, corresponding to its maximum recommended field rate. A control group (distilled water) and a group treated with a reference item (dimethoate) were also included in the study. The experiment on A. rhopalosiphi was performed according to the test method of Mead-Briggs et al. (2010), while the test on R. padi was conducted using the Institute’s own method. The mortality results prove that tebuconazole was toxic to the wasps and the aphids directly after the second treatment. It can be concluded that the recolonization of the wasp A. rhopalosiphi and the aphid R. padi on barley, treated with tebuconazole, can take place 7 days after the second treatment. The test fungicide negatively affected the wasp fecundity as the reduction in fecundity amounted to 27.3% when compared with the control group.