Galls formed by insects can act as sinks for nutrients and attract other herbivores to feed on gall tissues, which initiates interspecific competition, sometimes nurturing the herbivorous insects and restraining the gall-inducing insect, particularly when this competition is plant-mediated. Here, to our knowledge, we provide the first evidence of a close relationship between a gall insect, Hartigiola annulipes (Diptera: Cecidomyiidae), and a sap-sucking, Liothrips setinodis (Thysanoptera: Phlaeothripidae). The thrips were observed feeding on young H. annulipes galls, formed on the common beech (Fagus sylvatica) leaves during spring. Among randomly chosen beech trees, 100 current-year shoots were surveyed to determine the number of H. annulipes galls and the presence of thrips on the leaves. Our results show that L. setinodis specimens were found significantly more frequently on leaves infested by the galler than on uninfested leaves. The consequences of feeding thrips at the site of gall formation are not known yet, but it can be supposed that they are unfavourable to the gall insects and therefore could be beneficial for the host plant.