The terrestrial and omnivorous robber crab Birgus latro inhabits islands of the Indian Ocean and the Pacific Ocean. The animals live solitarily but occasionally gather at freshly opened coconuts or fructiferous arenga palms. By analyzing volatiles of coconuts and arenga fruit we identified five compounds, including Acetoin, which are present in both food sources. In a behavioral screen performed in the crabs’ habitat, a beach on Christmas Island, we found that of 15 tested fruit compounds Acetoin was the only volatile eliciting significant attraction. Hence, Acetoin might play a key role in governing the crabs’ aggregation behavior at both food sources.