Bromus secalinus L. is an annual speirochoric species growing in agrocenoses of winter crops. In the past, B. secalinus L. could be found throughout Poland. The regression of the species has caused it to be classified as a rare species threatened with extinction. The study was conducted in the 2007-2010 time period, in north-eastern Poland where a considerable increase in the number of sites and abundance of B. secalinus were observed. The plant material was collected from agrocenoses of winter crops, at 14 sites representing the habitat diversity of the species occurrence. Eight morphological features were taken into account when evaluating populations of B. secalinus. The morphological diversity of the populations of B. secalinus was correlated with different habitat conditions (type, pH of soil and cultivated plant). The populations growing on the sites with optimum trophic and moisture conditions as well as those populations growing in very poor conditions were found to differ the most. A comparison of B. secalinus was done and six groups of cluster similarities were found using the agglomeration method Electrophoretic analysis of seed storage proteins of B. secalinus L from the studied populations were performed using SDS-PAGE. Analysis revealed relatively high genetic polymorphism irrespective of whether genetic variation was compared between the populations from different sites or between the subpopulations from only site number 4. Three homologenous groups were formed which were related to the cultivated plant coverage, and partially, to the type of soil. In the investigated populations and subpopulations, protein bands of molecular weight from 8 to 68 kDa were observed. The SDS-PAGE method was found suitable for the assessment of genetic variation in populations of B. secalinus L from many sites as well as within the subpopulations from one site.