Linseed, one of the oldest cultivated crops, is again gaining in importance, mainly due to its nutritional benefits and biomedical applications. Therefore, it is expected that herbivores will also exist in greater abundance. Among them the flea beetle, Aphthona euphorbiae Schrank and Longitarsus parvulus Paykull are considered to be serious pests of flax grown for fibre and seeds in Europe. The aim of this study was to determine flax flea beetles’ abundance, species richness and seasonal dynamics on linseed grown at different densities. It was expected that linseed seeding density can significantly affect flea beetle populations. The experiment was carried out in Lower Silesia, Poland, from 2011 to 2013. A genetically modified type of linseed overproducing flavonoids was used. Flea beetles and the damages they caused were determined on plants and also a sweep net was used for the collection of adult beetles. During the three years of the study 15 species of flea beetles were identified from oil flax plants, with A. euphorbiae and L. parvulus being dominant. In terms of the total catch, the tendency was for beetle numbers to decrease with increasing plant density. Flax flea beetles feeding on linseed plants, irrespective of plant density, had two peaks of abundance. The first peak was lower and occurred in June, when plants were at the blooming stage. This peak was caused by overwintering adults who colonized crops in spring. The second, higher peak of abundance was recorded in the second half of July, when plants were at the ripening stage. This peak was formed by adults of the new generation. Each year, at the higher population peak of abundance, the flea beetles were most numerous on plants grown at the lowest density. There was one period, lasting either from mid-May to the first few days of June, or from the beginning of June to mid-June, during which the number of holes and damage on plants of each treatment were highest. During the three years of the study there were several cases of significantly higher numbers of flea beetle feeding symptoms on plants grown at the lowest density as compared to the medium and highest densities.