An external genetic input of Posidonia oceanica fruits dispersed by currents in the Ligurian Sea (Western Mediterranean) was investigated. During 2003-2004, when a massive fruiting event occurred, fruits were collected from plants at Monterosso al Mare (meadow) and compared with stranded fruits sampled in front of the meadow and downcurrent in Tuscany along 80 km of the coast. After their growth in culture, the plants were analysed using 10 random amplified polymorphic DNA (RAPD) molecular markers. Cluster analysis of similarity showed four distinct genetic populations. One group included parental plants from Monterosso and stranded fruits with the same genetic traits. The second group was formed by fruits stranded in the southern sector (Tuscany), and the third and the fourth groups were samples taken onshore, in front of the meadow, which appeared very different from the other two groups. Results evidenced the probability of the arrival of a new genetic population of nearly 40%, and it seems likely that the stranded fruits of external provenance did not come from the nearest P. oceanica meadows of Tuscany but were probably carried ashore by the Corsica current, as supported by an oceanographic analysis.