The studies were conducted in the closed static field experiment established in 1990 at the Production-Experimental Enterprise in Bacyny. The experiment was established on the medium typical grey-brown podzolic soil. The paper presents the results of 3 years of studies (2002-2004) on the role of Italian ryegrass in development of the communities of weeds in spring barley sown after different previous crops (potato, spring wheat, spring barley). Every year at the stage of tillering and before harvest of spring barley the composition of species in the community and density of individual species as well as additionally, during that later period, the biomass of weeds were determined. The number of species and the Shannon-Wiener diversity index were assumed as parameters for evaluation of the diversity of the weeds community. Additionally, the Simpson domination index was determined. Comparisons of communities were conducted applying the Sorensen similarity index. The intercrop of Italian ryegrass had minor influence on limiting the weeds in spring barley by reducing their numbers and biomass of the weeds. Its positive influence was more pronounced in case of cultivating spring barley after spring barley and after potato than after spring wheat. The communities of weeds in spring barley consisted of similar numbers of species and their share in the total population or biomass of phytocenoses was subject to only minor diversifications under the influence of the experimental factors. The population of Chenopodium album was characterised by the largest population and dry mass among the weeds. Additionally, in the spring, numerous sprouts of Thlaspi arvense and at the end of spring barley vegetation a significant density of Fallopia convolvulus were recorded. On the other hand, as concerns "shapeliness" Fallopia convolvulus, Stellaria media and Sonchus arvensis were the complementary taxa. Before spring barley harvest the domination indexes were higher than in the spring and the diversity indexes lower. Presence of Italian ryegrass and choice of previous crop had minor influence on changes in the values of indexes at the analysed times of evaluation of weeds in spring barley. The communities of weeds were more similar in their composition of species and populations of individual species than in their dry mass.
