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The frequency of Posidonia oceanica past flowering events in a north-western Mediterranean area was examined using a reconstructive age technique. We also investigated possible relationships among shoot age and occurrenceof flowering. Seven flowering events were recorded over the 14-year period examined. Considerable variations in inflorescence frequency occurred among years (0-25.3%), with two years of abundant flowering (1989 and 1993) separated by several years with little or no flowering. Only 2% of the shoots sampled flowered twice; contrary to previous opinion, the interval among two successive flowerings was considerably shorter than 10 years, varying from 1 to 4 years. The minimum reproductive age of shoots was 6 months. The probability of flowering increased with age until 6 years of age, and decreased thereafter. Both the number of leaves and rhizome segments produced by a shoot during a flowering year did not differ significantly from those produced in the years preceding and following a flowering event, suggesting that the cost of flowering could not affect short-term growth of shoots. On the basis of our results we can speculate that the maximum sexual reproductive output for P oceanica in the area studied was about 340-460 seeds m - 2 in 1994.