The seagrass Posidonia oceanica plays an important role as habitat for invertebrates. A correlative study was done in El Campello (SE Spain) to understand the relationships between assemblage structure of crustaceans and modification of P. oceanica structure caused by bottom trawling. We compared the changes of community structure of the most important groups of crustaceans, amphipods and decapods with changes of cover, density, detritus and rhizome fragments. Sampling was carried out in four control and four impact locations, along a gradient of degradation (17 m depth, August 1992). Multivariate analysis was used for community data (nm-MDS) and P. oceanica structure (PCA). Both groups showed a similar number of species. The amphipod community showed a clear pattern of community changes with significant differences between impact and control locations (global R = 0.298, p<0.001) and with significant correlation with P. oceanica structure (global r = 0.21, p<0.05). Detritus (fine and coarse) and rhizome fragments had the highest influence on amphipod distribution. Spatial variability inside of locations was very high. Few species (Dexamine spiniventris, Lysianassa longicornis or Ampelisca spp.) were responsible for disimilarity among locations. Decapods did not show a correlation with the structure of P. oceanica. The amphipod community seems to be a strong indicator of the ecological implications of seagrass degradation.