Abstract.The frequency of parasitism of free ending prop roots of the red mangrove,Rhizophora mangle,by the wood boring isopodSphaeroma terebrans,increased with increasing distance of the endings to established roots. In a field experiment, in which isolated endings were, surrounded by wooden sticks, the endings were colonized byS. terebransat a lower rate than control endings, thus indicating that the wooden structures in the vicinity of the ending tended to reduce colonization rate byS. terebrans.It is concluded thatS. terebransmay cause mangrove root density to increase by colonizing peripheral endings more readily than endings which are near established roots.