TreeView is a simple, easy to use phylogenetic tree viewing utility that runs under both MacOS (on Apple Macintosh computers) and under Microsoft Windows on Intel based computers, the two most common personal computers used by biologists. Some phylogeny programs, such as PAUP (Swofford, 1993) and MacClade (Maddison and Maddison, 1992) already provide excellent tree drawing and printing facilities, however at present these programs are restricted to Apple Macintosh computers. Furthermore, they require the user to load a data set before any trees can be displayed which is inconvenient if the user simply wants to view the trees. More portable programs, such as DRAWGRAM and DRAWTREE in the PHYLIP package (Felsenstein, 1993) can run on both MacOS and Windows computers, but make little, if any use of the graphical interface features available under those operating systems. TreeView runs as a native application on either MacOS or Windows computers, enables the user to use the standard fonts installed on their machine, their printer, and supports the relevant native graphics format (PICT and Windows metafile) for either creating graphics files or pasting pictures to other applications via the clipboard. The program also supports standard file operations, such as 'drag and drop' whereby dragging a file's icon onto the program opens that file. TreeView can read a range of tree file formats (see below) and can display trees in a range of styles (Fig. 1). Additional information, such as edge lengths and internal node labels can also be displayed. The order of the terminal taxa in the tree can be altered, and the tree can be rerooted. If the tree file contains more than one tree the user can view each tree in turn.