A hashing function is perfect if it does not create any overflow records. the use of perfect hashing functions has previously been studied only for small static sets stored in main memory. in this paper we describe a perfect hashing scheme for large external files which we are currently investigating the scheme guarantees retrieval of any record in a single disk access. this is achieved at the cost of a small in-core table and increased cost of insertions we also suggest a policy for limiting the cost of insertions and we study the tradeoff between expected storage utilizations size of the internal table and cost of insertions under this policy the results obtained so far are very promising they indicate that it may indeed by possible to design practical perfect hashing schemes for external files based on the suggested approach.