The A00 extension is given to a file which is one of the elements of a multi-part archive of the ARJ format. It is a data compression format and a program for handling such archives. It was created by Robert K. Jung.

A00 is a file that stores the 2nd part of an archive composed of more than one file. Dividing the archive into several smaller files allows you to place them on smaller data carriers. This was especially important since this format was developed at a time when floppy disks were widely used. The first archive file is ARJ, then there are A00, A01, A02, etc. However, this solution was superseded by RAR and ZIP formats.