One way to provide fault isolation among cooperating software modules is to place each in its own address space. However, for tightly-coupled modules, this solution incurs prohibitive context switch overhead. In this paper, we present a software approach to implementing fault isolation within a single address space. Our approach has two parts. First, we load the code and data for a distrusted module into its own fault domain, a logically separate portion of the application's address space. Second, we modify the object code of a distrusted module to prevent it from writing or jumping to an address outside its fault domain. Both these software operations are portable and programming language independent. Our approach poses a tradeo relative to hardware fault isolation: substantially faster communication between fault domains, at a cost of slightly increased execution time for distrusted modules. We demonstrate that for frequently communicating modules, implementing fault isolation in software rather than hardware can substantially improve end-to-end application performance.