Numbers of banner-tailed kangaroo rats, Dipodomys spectabilis, have declined sharply in some but not all populations monitored in southeastern Arizona over the past 20 years. We describe concurrent changes in vegetation and report the results of microhabitat manipulation experiments in which we removed broom snakeweed, Gutierrezia sarothrae, from 1.00-ha (pilot) or 0.56-ha (replicate follow-up) plots. D. spectabilis became extinct on control plots, but populations remained stable on plots where snakeweed was removed. On a larger scale, declines in numbers of kangaroo rats coincided with increases in density of woody plants. The data substantiate the preferences of this species for structurally open microhabitats and document that survival rates are higher in areas that are more open. Large kangaroo rat species like D. spectabilis are often regarded as keystone species, and our results indicate that they are vulnerable to grassland degradation.