As part of the US/International Program Grassland Biome study in northeastern Colorado, six 1—ha plots of shortgrass prairie were manipulated for 6 yrs (1971—1976) through the addition of N, water, or both. Two additional 1—ha plots served as controls. Microtus ochrigaster invaded the 2 N + water treatment plots in 1971, and increased in annual peak densities (occurring in late summer or fall each year) linearly through the completion of this study in 1976. These vole populations exhibited no multiyear cycles in population density usually common in microtines, but they did undergo exaggerated annual fluctuations in density. Patterns in reproduction, mortality, sex ratios, body weight, annual population fluctuations, and dispersal were analyzed and compared with those of populations reported in the literature to exhibit 2—4 year cycles in population density. Only small differences in these parameters were found between the Pawnee populations and multiyear cyclic populations. The N + water plots represented "islands" of suitable vole habitat in a "sea of shortgrass prairie," which is apparently inhospitable for voles. Thus, these semi—isolated populations of noncyclic (or annually cyclic) prairie voles allowed us to speculate on several aspects of the Chitty/Krebs theory of self—regulation in microtine populations. This theory (as modified by Krebs) implicates emigration as an important determinant of 2—4 year cycles. We propose that the importance of emigration is in regulating population size while immigration is probably responsible for the existence of the cycle.