Summary: A Tree Viewer (ATV) is a Java tool for the display and manipulation of annotated phylogenetic trees. It can be utilized both as a standalone application and as an applet in a web browser. Availability: ATV is available via WWW at http://www.genetics.wustl.edu/eddy/atv/ and via FTP at ftp://ftp.genetics.wustl.edu/pub/eddy/software/forester.tar.Z Contact: eddy@genetics.wustl.edu