Two populations of Avena barbata have been sampled intensively and allelic frequencies have been determined on a fine grid for six enzyme loci. The distribution of alleles among microniches within the location occupied by one of these populations (CSA-1) was correlated with environment following patterns of macro- and microdifferentiation previously described in California. Multilocus analyses revealed striking gameticphase disequilibrium for both linked and unlinked loci, and also that two balanced five-locus gametic types, one associated with mesic conditions and the other with more xeric conditions, are in substantial excess at the CSA-1 location. Evidence is presented that these two gametic types are maintained by selection acting on two coadapted combinations of alleles, held together by restriction of recombination resulting from linkage and/or inbreeding. Gametic-phase disequilibrium featuring different allelic complexes were found in each of three microniches in the second population (Geyserville), which occupies a site that is environmentally atypical for A. barbata.