Motivation: Real evolutionary data often contain a number of different and sometimes conflicting phylogenetic signals, and thus do not always clearly support a unique tree. To address this problem, Bandelt and Dress (Adv. Math., 92, 47-05, 1992) developed the method of split decomposition. For ideal data, this method gives rise to a tree, whereas less ideal data are represented by a tree-like network that may indicate evidence for different and conflicting phylogenies. Results: SplitsTree is an interactive program, for analyzing and visualizing evolutionary data, that implements this approach. It also supports a number of distances transformations, the computation of parsimony splits, spectral analysis and bootstrapping. Availability: There are two versions of SplitsTree: an interactive Macintosh version (shareware) and a command-line Unix version (public domain). Both are available from: ftp://ftp.uni-bielefeld.de/pub/math/splits/splitstree2. There is a WWW version running at: http://www.bibiserv.techfak.unibielefeld.de/splits. Contact: huson@mathematik.uni-bielefeld.de.