A file system for a portable data carrier provides improved security for files which support multiple applications, from custom repertory dialing to storage of individual medical and/or banking records. Although the portable data carrier looks and feels much like an ordinary credit card, it includes a computer and an electrically erasable programmable read-only memory. Power for operation of the portable data carrier is provided from an associated station via a reader/writer. The reader/writer also couples data between the data carrier and the associated station. The applications reside in multiple files in memory on the portable data carrier. Appropriate application software residing in the station, when accompanied by an appropriate password, enables the retrieval and modification of these files. A separate password is required for gaining access to each of designated levels of interaction between the portable data carrier and the associated station. Additional restrictions such as requiring an additional password for writing to a file and also allowing a user logged in at a particular security level to only append information to a file may be imposed in accordance with file security on the portable data carrier. And since each of the files may have its own security requirements, multiple applications may exist on the portable data carrier without conflict or confusion.