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Tree snails of the family Partulidae have declined on Guam since WWII. One species, indigenous to the western Pacific, Partula radiolata, is still locally common along stream courses in southern areas of the island. The Mariana Island endemic Samoana fragilis is present but not found in abundance anywhere on Guam. Partula gibba, another Mariana endemic, is currently known only from one isolated coastal valley along the northwestern coast, and appears to be in a state of decline. The Guam endemic Partula salifana was not found in areas where it had been previously collected by earlier researchers, and is thus believed to be extinct. The decline and extinction of these snails are related to human activities. The single most important factor is likely predation by snails that were introduced as biologicl control agents for the giant African snail, Achatina fulica. The current, most serious threat is probably the introduced flatworm Platydemus manokwari. This flatworm is also the likely cause of extinctions of other native and introduced gastropods on Guam and may be the most important threat to the Mariana Partulidae.