(Uploaded by Plazi for the Bat Literature Project) Twenty-two summer colonies of the endangered gray bat, Myotis grisescens, were censused in 1968-70 and 1976. A conservative estimate revealed a 54% decline in that time period and a 76% decline from known past maximum population levels. A strong association between decline and disturbance by people in caves was observed. Some major colonies disappeared entirely within the 6-year period. Gray bats are restricted to caves year-round and, due to specific temperature and foraging habitat requirements, they aggregate in large colonies in fewer than 5% of available caves. Management requires that the 9 known hibernation caves receive immediate protection, followed by protection of the most important summer caves used by bats from each protected winter cave. Adequate protection may prove impossible unless accompanied by public education. Environmental disturbances such as pesticide contamination, water pollution and siltation, and deforestation may pose serious threats and require further investigation.