A functional data-parallel language called BSML was designed for programming Bulk-Synchronous Parallel algorithms, a model of computing which allows parallel programs to be ported to a wide range of architectures. BSML is based on an extension of the ML language with parallel operations on a parallel data structure called parallel vector. The execution time can be estimated. Dead-locks and indeterminism are avoided. For large scale applications where parallel processing is helpful and where the total amount of data often exceeds the total main memory available, parallel disk I/O becomes a necessity. In this paper, we present a library of I/O features for BSML and its formal semantics. A cost model is also given and some preliminary performance results are shown for a commodity cluster.