X-ray data from the ROSAT satellite will be distributed to Guest Observers on FITS tapes. The precise contents and format of the tape were chosen bearing the following issues in mind: 1. The primary data component for an X-ray observation is a list of photon events, and the user requires additional data for selection and analysis of the primary data. The introduction of the FITS bintable extension has provided a standard format that makes the storage of photon events plausible and efficient. 2. The photon events have associated properties, such as position, time, energy, quality, but the precise list of properties is experiment-dependent. This issue has been addressed by defining minimum elements for each record type, but allowing self-defining record extensions, specific to individual projects. 3. Filtering and selection of photons will be a user's first step in the scientific analysis of the data. This issue has been addressed by the inclusion of multiple filters to permit the storage of all photon events, while providing a simple mechanism for extracting a nominal subset that includes only good events. These solutions in defining the contents and formats of the FITS files should be useful for other event-based data sets.