Between August and October of 2015, we found five fresh carcasses of wild Malayan flying lemurs (Galeopterus variegatus) in Pangandaran Nature Reserve, West Java, Indonesia. Such mass mortality of this species has never been reported though several researchers in SE Asia had reported cases of finding fresh carcasses intermittently. We tried to address causal factor(s) of the rare event. We reviewed own data on temperature, monthly rainfall (mm), plant phenology (of mature/young leaves, flowers, and mature/young fruits) and compared among 1) before, 2) during, and 3) after the mass mortality event. There were no clear relationships between temperature and the mortality event, while phenology score and rainfall during the mass mortality periods were significantly lower than those in other phases. Abnormal weather (drier environment) and/or food scarcity, therefore might be one reason for their deaths. Another likely reason of the mass mortality is recent environmental changes caused by humans. Since the development of a resort area around the study site started at the end of 2014, many tall trees, whose leaves and fruits form diets of the folivorous animals, including the flying lemurs, at the northern part of the study area were cut down. In the same period, one group of leaf monkeys started invading human settlements in search of food, and several deaths occurred. In order to test the effects of these factors, we need to conduct systematic monitoring of the environmental fluctuation in the study site.