What is the A00 file format?
An .a00 file is the second volume of a multi-volume ARJ archive. ARJ (Archived by Robert Jung) was a popular DOS-era compression format introduced in 1990, widely used throughout the 1990s to distribute software across multiple floppy disks. When an ARJ archive is split into parts, the first part uses the .arj extension while subsequent volumes follow the sequence .a00 (volume 2), .a01 (volume 3), .a02 (volume 4), and so on. This naming is non-intuitive compared to formats like RAR, where .r00 is always volume 2 after .rar.
Like all ARJ volumes, an .a00 file begins with the two-byte ARJ header magic 60 EA. The format uses an LZH-derivative compression algorithm, optional DES password protection, and CRC-32 checksums for every file header and data block. Individual files that span volume boundaries carry continuation flags in their headers. Extraction must always start from the first volume (.arj) and process all subsequent parts in order.
The multi-volume feature was essential when the 1.44 MB floppy disk was the primary software distribution medium. Modern tools including 7-Zip and WinRAR still support ARJ as a legacy format, so .a00 files from old DOS archives can still be extracted today.
ARJ has been superseded for all new work by RAR, 7-Zip, and ZIP-based split archives, which offer better compression ratios and wider tool support.
Security & safety
RISK: LOWARJ archives can contain any file type; the same risks apply as with any archive (executable payloads, path traversal exploits in old extractors). Use an up-to-date extractor. ARJ is an obsolete format; encountering it in the wild typically means legacy software, abandonware, or old backups rather than active malware distribution.
Format details
in a nutshellPrograms that open A00 files
Windows4 apps
WinRAR
Freemium Open the .arj first part in WinRAR; it prompts for subsequent volumes from the same folder. 
7-Zip
Open-source Place all parts (.arj, .a00, .a01…) in one folder. Right-click the .arj file > 7-Zip > Extract Here. 7-Zip reads subsequent volumes automatically. 
ARJ (command-line)
Free arj e archive.arj - extracts all volumes automatically; last release v3.15 (2008). 
Linux2 apps
7-Zip (p7zip / 7-Zip native)
Open-source 7z e archive.arj - extracts multi-volume ARJ from command line. Technical details
deep spec| Format type | ARJ multi-volume archive segment (volume 2 of a split set) |
| Magic bytes | 60 EA at offset 0 (ARJ local header signature) |
| Byte order | Little-endian |
| Encoding | Binary |
| Compression algorithm | LZH-derivative (ARJ proprietary); optional GZIP in newer versions |
| Encryption | Optional DES password protection (arj -g flag) |
| Checksum | CRC-32 per file header and per compressed data block |
| Typical split size | 1.44 MB (floppy-sized) or user-defined at archive creation |
| Volume naming sequence | `.arj` (vol 1) → `.a00` (vol 2) → `.a01` (vol 3) → `.a02` (vol 4) |
| Max files per archive | 65,535 (14-bit count field in ARJ header) |
| Extraction requirement | Must start from first volume (`.arj`); volumes must be processed in order |
| Platform | DOS, Windows, Linux, OS/2 |
| ARJ version range | v1 (1990) through v3.15 (2008); version encoded in main archive header |
| Status | Effectively obsolete; supported as legacy format by 7-Zip and WinRAR |
| Released | 1990 (ARJ 1.0 by Robert Jung) |
| Latest version | ARJ 3.15 (2008) |
| Specification | www.fileformat.info |
A00 conversions
Community Q&A
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