The Posidonia Monitoring Network (PMN) is based on a biological indicator, the seagrass Posidonia oceanica. It was set-up in 1984 and concerns 33 survey sites located along the Provence and French Riviera coasts, at either the upper or lower limits of the seagrass meadows. Monitoring the lower limit is carried out by positioning permanent cernent markers along this limit and by photographing these markers and studying several P. oceanica vitality parameters every 3 years. Monitoring the upper limit is based on the same rnethods, completed with aerial photographs. Between 1984 and 1999, the percentage of sites at which the P. oceanica meadow was seen to be in expansion increased from 21 to 42%. This is consistent with improvement in the percentage of waste waters which are processed in treatment plants, less than 10% in the early 1980s and close to 100% today.