Survey data on Posidonia oceanica was gathered by volunteer divers during spring and summer 2005 in Cala Montgo, a sheltered bay and popular anchorage, near L’Escala within the Bay of Roses. Three sites were selected for comparison and 64 square metres were sampled in total. Data was recorded on P. oceanica density, and the diversity of flora and fauna, within square metre samples. Whilst conclusions cannot be drawn about causality in relation to differences in P. oceanica density, or the presence of key species, between study sites; the measures obtained provide a baseline against which to make comparisons in future years. It is planned to continue the process of Posidonia oceanica monitoring on an annual basis in the light of the threat to Posidonia oceanica posed by the building of a 130,000 square metre fish farm in the Bay of Roses.