Current approaches to parallel I/O demand extensive user effort to obtain acceptable performance. This is in part due to difficulties in understanding the characteristics of a wide variety of I/O devices and in part due to inherent complexity of I/O software. While parallel I/O systems provide users with environments where large datasets can be shared between parallel processors, the ultimate performance of I/Ointensive codes depends largely on the relation between data access patterns and storage patterns of data in files and on disks. In cases where access patterns and storage patterns match, we can exploit parallel I/O hardware by allowing each processor to perform independent parallel I/O. To handle the cases in which data access patterns and storage patterns do not match, several I/O optimization techniques have been developed in recent years. Collective I/O is such an optimization technique that enables each processor to do I/O on behalf of other processors if doing so improves the overall performance. While it is generally accepted that collective I/O and its variants can bring impressive improvements as far as the I/O performance is concerned, it is difficult for the programmer to use collective I/O effectively. In this paper, we propose and evaluate a compiler-directed collective I/O approach which detects the opportunities for collective I/O and inserts the necessary I/O calls in the code automatically. An important characteristic of the approach is that instead of applying collective I/O indiscriminately, it uses collective I/O selectively, only in cases where independent parallel I/O would not be possible. We have implemented the necessary algorithms in a source-to-source translator and within a stand-alone tool. We have conducted several experiments using a..