Field surveys were conducted to evaluate the occurrence of the isopod borer Sphaeroma terebrans (Crustacea) in aerial roots (prop roots) of the red mangrove Rhizophora mucronata on several different spatial scales (m to 100 km) in East Africa. In 6 out of 17 sites studied in Kenya and on Zanzibar Island, Tanzania, no signs of the isopods were found. When the isopods were present the frequency of infestation was high. Trees in muddy substrates in the lower intertidal, in particular at fringing channels or the open sea, showed high prevalence and intensity of infestation, with large part of their roots damaged or dead. Trees at the upper range of Rhizophora, in sandy and muddy areas, showed no signs of isopod infestation. This pattern recurred in mangrove forests on large spatial scales and there was no indication that island forests differed from the mainland forests. This indicates that sediment characteristics, vertical height in the tidal zone, and direct exposure to incoming water are the major factors controlling the abundance of S. terebrans. The isopod may play an important role in determining the lower intertidal limits of R. mucronata. Trees with numerous dead or nongrowing roots, as result of Sphaeroma attack, are likely to tumble due to a lack of root support and this is most likely to occur along channels at the lower, muddy intertidal. Tumbled trees were frequently observed along channels in the lower, muddy intertidal, but rarely in the mid or high intertidal. Implications for management of mangrove forests are discussed.