The information on this page applies to Pro Tools 8 and earlier. Pro Tools 9 requires Mac OS X 10.6.2 (or higher), and is not compatible with earlier versions of Mac OS X:

Pro Tools 9 Information

System Requirements and Compatibility with Pro Tools 9.0 Software:
System Requirements and Compatibility with Pro Tools|HD and Pro Tools|HD Native Systems with Pro Tools HD 9.0 Software:

Mac OS X Requirements with Pro Tools 6, 7, and 8

Additional Information