The current intensive anthropogenic activity has exerted a significant effect on the formed biotypes. This resulted in marked changes in composition and structure of segetal flora. Qualitative and quantitative composition of segetal flora in a sugar beet stand and its changes after one crop rotation (7 years) is analysed in this article. Research was carried out at the Rumokai Research Station of the Lithuanian Institute of Agriculture (Vilkaviškis distr. Klausučiai vicinity) in 2000 and 2008. The soil of the experimental site is medium heavy loam on clay. Segetal plants were counted in the sugar beet stand before the first herbicide application, and their species composition and number per m2 were established. In 2000 the prevalent weed species in the sugar beet stand were Lamium purpureum L., Galium aparine L., Chenopodium album L. In total, 17 weed species were identified in the crop stand. In 2008 species composition of segetal plants and quantitative parameters were similar. Assessment of ecological groups of segetal plants according to their demand for soil pH showed that in 2000 and 2008 most of the weed species belonged to the group of indifferent plants that grow in soils with a wide range of acidity. According to soil nitrogen demand, in 2000 most of the weed species belonged to the N6 group. In 2008 the weeds distributed almost evenly within N4 group according to the demand for soil nitrogen.