Abstract Motivation: To produce an open and extensible file format for DNA trace data which produces compact files suitable for large-scale storage and efficient use of internet bandwidth. Results: We have created an extensible format named ZTR. For a set of data taken from an ABI-3700 the ZTR format produces trace files which require 61.6% of the disk space used by gzipped SCFv3, and which can be written and read at greater speed. The compression algorithms used for the trace amplitudes are used within the National Center for Biotechnology Information (NCBI) trace archive. Availability: Source code is available from ftp://ftp.mrc-lmb.cam.ac.uk/pub/staden/io_lib/io_lib.tar.gz. A complete format description can be found at http://www.mrc-lmb.cam.ac.uk/pubseq/ztr.html. Test data is available from ftp://ftp.mrc-lmb.cam.ac.uk/pub/staden/io_lib/test_data. Contact: jkb@mrc-lmb.cam.ac.uk * To whom correspondence should be addressed.