In a series of papers on the population biology of Avena fatua L. (common wild oat) and A. barbata Brot. (slender wild oat) the patterns of genetic variation in natural populations of these species were discussed in relation to their ecology and evolution (Jain and Marshall, 1967; Marshall and Jain, 1968; Jain, 1969 et seq.). Studies based on morphological and isoenzymatic markers revealed marked differences in population structure both between the two species and among populations within each species. Further, our results suggest that the differences in the population structure of the two species are a reflection of their different adaptive strategies; A. barbata is phenotypically more flexible than A. fatua and seems to rely more heavily on this mechanism in adapting to the variety of habitats it occupies. In addition, it would appear that differences among populations within a species reflect, in part, differences in the heterogeneity of the habitats they occupy although other factors such as variation in outcrossing rates, species composition and diversity of the community are undoubtedly involved. However, further studies on a wider range of species are necessary to establish the generality of these observations and to further elucidate the relationships between the ecology and genetics of grassland species. We have therefore undertaken a series of studies on the ecogenetics of Bromus species in central California. As with our Avena studies, emphasis will be given to the description of genetic variation, population regulation, species interactions and their place in the comrnunity. Several Bromus species, including B. mollis, B. rubens, B. sterilis, and B. rigidus, occur in the California annual type vegetation and between them they account for more than 20% of the herbaceous cover (Talbot et al., 1939). Of these, B. mollis is the most abundant in central California, and is often a dominant species, particularly under moderate grazing and favorable soil conditions. McNaughton (1969) recently discussed the importance of B. mollis in the functional and floristic diversity of the grassland vegetation while Love (1961), among others, has emphasized its desir1 This work was supported in part by grants from the National Science Foundation (GB-8627 and GB-3246) and the National Institutes of Health (GM-10476). 2 Present address: Division of Plant Industry, CSIRO, Canberra, Australia.