Analysis of dispersion patterns for a population of the California vole (Microlus californicus) within its preferred habitat indicated higher densities associated with patches of a perennial grass (Elymus cinereus) than with the dominant annual (Bronuis rigidus). Although this pattern occurred throughout the year, the only demographic differences between subpopulations occurred during the warm, dry summer. More adult (heavier) voles were found in Elymus during August, a result of improved survival and growth. The greater availability of preferred food (green forage) in Elymus patches suggested that the demographic improvement resulted from better nutrition. Such patches serve as refugia for voles when populations are low. thus assuming increased significance.