CMZ File

What are CMZ files and how to open them

Are you having problems opening a CMZ file or are you simply curious about its contents? We're here to explain the properties of these files and provide you with software that can open or handle your CMZ files.

What is a CMZ file?

CMZ files have multiple uses, and CMZ Compressed Archive is one of them. Read more about the other uses further down the page.

CMZ Compressed Archive

CMZ Compressed Archive is a proprietary archive format used in certain Lotus software installer packages, most notably some versions of Ami Pro. It bundles multiple files in a compressed form for distribution or installation purposes, acting much like an installer archive. ezyZip identifies it as being known only from Lotus Ami Pro distribution disks, and suggests it does not appear elsewhere in regular usage.

Archive creation and handling appear limited to Lotus's tools or related installer utilities. Member files within a CMZ archive are compressed using the PKWARE Data Compression Library methods. Though no official Lotus specification for CMZ is publicly documented, tools like deark support it experimentally, indicating some understanding of its internal structure for decompression.

Files in this format can be identified by a signature at their start. They begin with the ASCII byte sequence "Clay" (bytes 0x43 0x6C 0x61 0x79) at offset zero.

How to open CMZ files

Important: Different programs may use files with the CMZ file extension for different purposes, so unless you are sure which format your CMZ file is, you may need to try a few different programs.

While we have not verified the apps ourselves yet, our users have suggested four different CMZ openers which you will find listed below.

Last updated: August 31, 2026

All known file formats using extension .CMZ

While CMZ Compressed Archive is a popular type of CMZ-file, we know of 2 different uses of the .CMZ file extension. Different software may use files with the same extension for different types of data.

Compressed Archive

Compressed Archive files are installer archive files produced by Lotus installers for packaging software components in a compressed bundle. They serve to gather multiple files - program modules, resources, configuration scripts - into a single compressed archive so that installers can distribute and unpack them more efficiently. These archives are created when Lotus distributes software (such as Ami Pro or similar Lotus product(s)), and are unpacked by the installer during setup.

Its main use is inside Lotus setup routines: the installer writes the archive file on distribution media or packaging, and the Lotus installer program reads it to extract the payload files during installation. The archive format bundles together many files in compressed form, allowing reduction in disc space and simpler management compared to shipping individual files separately.

Files are identifiable immediately by their content: at offset zero the first four bytes read "Clay" in ASCII (hex 43 6C 61 79), which is the internal signature marking this format.

Various apps that use files with this extension

These apps are known to open certain types of CMZ files. Remember, different programs may use CMZ files for different purposes, so you may need to try out a few of them to be able to open your specific file.

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