Utah juniper (Juniperus osteosperma) has greatly expanded its historical range in the western United States. Management plans for the species have focused on curtailing its encroachment into sagebrush and grassland communities. These plans often include burning or other methods of elimination. These methods may result in subdivision or fragmentation of existing juniper stands. We initiated a study at Dugway Proving Ground, a U.S. Army facility, to examine the effects of fragmentation on the reproductive success of Utah juniper in isolated populations. We used enzyme electrophoresis to quantify genetic variability in isolated populations. We also determined population reproductive success by examining juniper fruits for evidence of seed abortion and/or presence of insect parasites. We compared reproductive and genetic variability in isolated populations at Dugway to 2 nonisolated and encroaching Utah juniper populations. The Dugway populations exhibited reduced seed set due to high seed abortion and/or insect seed parasitism, and a loss of genetic variability in comparison to the nonisolated populations. Additionally, there was a significant correlation between reproductive success and genetic variability.