In this paper, we propose a strategy for implementing parallel-I/O interfaces portably and efficiently. We have defined an abstract-device interface for parallel I/O, called ADIO. Any parallel-I/O API can be implemented on multiple file systems by implementing the API portably on top of ADIO, and implementing only ADIO on different file systems. This approach simplifies the task of implementing an API and yet exploits the specific high-performance features of individual file systems. We have used ADIO to implement the Intel PFS interface and subsets of MPI-IO and IBM PIOFS interfaces on PFS, PIOFS, Unix, and NFS file systems. Our performance studies indicate that the overhead of using ADIO as an implementation strategy is very low.