Posidonia oceanica (L.) Delile meadows are long-lived systems that persist in the field for millennia. The age and size of single clones have not been clearly assessed, nor has the withinmeadow dispersal of sexual propagules and neighbourhood size. The present study describes the genetic structure of an ancient and large P. oceanica meadow, extending from 3 to 33 m depth, that has been analyzed utilizing 13 variable microsatellite loci. A total of 180 single shoots (ramets) was sampled in 21 areas selected at nodes of a 160 × 400 m grid superimposed on the meadow. For each area, shoots were collected at a reciprocal distance of 1 to 5 m. The number of distinct genotypes was assessed for each sampling area and mapped on the grid using a kriging technique. Neighbourhood size and meadow-scale gene flow were assessed by means of autocorrelation analysis. Data indicate that the meadow is composed of a number of distinct clones, some of which might be hundreds of years old. Different sample groups were identified within the meadow by means of a Bayesian approach. The pattern of genetic diversity is not always related to shoot density, but it increases in the deepest stand, where density is lower. Spatial autocorrelation analysis showed a significant correlation up to 70 or 40 m, considering all samples or only distinct genotypes, respectively. The P. oceanica meadow analyzed seems to result from initial recruitment events and active clonal growth of originally established genotypes.