Incident light available to a meadow of the seagrass Posidonia sinuosa Cambridge and Kuo in Princess Royal Harbour, Western Australia, was reduced by 80-99% for 148 days using shadecloth, starting in summer. Shading reduced the density of leaf-bearing shoots, leaf density, primary productivity and the leaf production per shoot. Leaf width was unaffected by shading. Recovery of the seagrass meadow was assessed from differences in the above variables among controls and treatments, measured on separate occasions over 245 days after removal of the shadecloth. Leaf production per shoot and leaf length of active shoots recovered rapidly after shading while shoot densities did not, being significantly lower than controls at the end of the study. Longer periods of shading produced greater reductions in shoot density and productivity. By the end of the study, shoot density and productivity of seagrasses shaded for 307 days were each about one-tenth that of unshaded seagrasses while productivity of seagrasses shaded for 393 days was less than 8% that of unshaded seagrasses. Such responses suggest that reduction in incident light at the levels used in this study would lead to collapse of the seagrass meadow within 2 years.