Abstract Summary: Estimating Phylogenies of Species (EPoS) is a modular software framework for phylogenetic analysis, visualization and data management. It provides a plugin-based system that integrates a storage facility, a rich user interface and the ability to easily incorporate new methods, functions and visualizations. EPoS ships with persistent data management, a set of well-known phylogenetic algorithms and a multitude of tree visualization methods and layouts. Implemented algorithms cover distance-based tree construction, consensus trees and various graph-based supertree methods. The rendering system can be customized for, say, different edge and node styles. Availability: Executables and source code are available under the LGPL license at http://www.bio.informatik.uni-jena.de/epos. Contact: [email protected] Supplementary information: The homepage contains tutorials and documentation for both users and programmers who want to develop plugins and extensions.