Citizen science is a powerful way to survey the distribution of alien species in a broader area and is effective in promoting public awareness. I conducted a citizen science project to survey the distribution of the alien Decollate Snail Rumina decollata in Osaka, central Honshu, Japan. This species has spread over the Prefecture in recent years. As a rst step I set up 'train station survey' in which each volunteer selects a train station in Osaka and looks for the snail on foot for 1 hour

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... n a 1-km radius from the station. This survey revealed that the snail inhabits mainly the Senboku area (the northern part of southern Osaka). The next step aimed to investigate in greater detail the snail's distribution in the Senboku area using the following methods: 1) Grid survey: each volunteer selects a focal grid (conforming to the third-order-unit grid de ned by the Statistics Bureau of Japan, ca. 1 km square) and looks for the snail within the grid for 1 hour. 2) Gathering observations from residents: iers are handed out to municipal elementary school children and library visitors in the Senboku area, or a social networking service is used to gather snail observations from the residents. Integration of the results of these surveys revealed that the Decollate Snail occurred in fty-one third-order-unit grids in Osaka Prefecture at the end of November 2019. Information from residents indicated that the snail had been introduced between 2000 and 2010 in Osaka and that one probable dispersal route was via gardening or agricultural materials such as soil or seedlings. The train station survey is an effective way to screen a highdensity area. Observations were provided continuously for four months by residents who had found out about this project from the iers distributed to the elementary school children. This suggests that iers in schools may be effective in collecting observations of alien species.