Genetic and environmental components were analysed in 32 Argentinian populations of Bromus catharticus. The research was based on 39 vegetative and reproductive characters. Constancy (rc) and heritability (h2) ratios were calculated. ANOVAS showed differences between populations for 14 traits, most of them reproductive. Total phenotypic variation was mostly due to the environmental component. Microfloral attributes showed the highest values of rc and h2. The traits average length of the spikelets (LS), average number of florets per spikelet (NFS), and lemma length (LL), which simultaneously reach rc values higher than 1 and h2 values higher than 0.60, could be considered useful in systematic studies. Leaf, stem, and some reproductive characters, linked to propagule production, had plastic responses. However, traits associated with size and shape of propagules and spikelets remained constant. Results suggest that a double strategy is operating: plasticity in some traits (to give greater adaptability), and constancy in other traits related to species stability that are of systematic significance.Facultad de Ciencias Agrarias y Forestales