Summary: PhyloWidget is a web-based tool for the visualization and manipulation of phylogenetic tree data. It can be accessed online or downloaded as a standalone application. A simple URL-based API allows databases to easily link to and customize PhyloWidget for interactively viewing medium- to large-sized trees. Availability: PhyloWidget is available for online use or download at http://www.phylowidget.org/. Its source code is released under the GNU General Public License. Contact: phylowidget@treebase.org 10.1093/bioinformatics/btn235