FASconCAT is a user-friendly software that concatenates rapidly different kinds of sequence data into one supermatrix file. Output files are either in FASTA, PHYLIP or NEXUS format and are directly loadable in phylogenetic programs like PAUP *, RAxML or MrBayes. FASconCAT can handle FASTA, PHYLIP and CLUSTAL formatted input files in one single run. It provides useful information about each input file and the concatenated supermatrix. For example, the program provides the range information of each concatenated gene (partition) and delivers a check list of all concatenated sequences (taxa). Information about the base composition of single input files and the resulting supermatrix is supplied for nucleotide data. For given structure strings (e.g. secondary structures) it displays single unpaired (loop) and paired (stem) positions after the concatenation process. Optionally, FASconCAT generates NEXUS files of concatenated sequences, either with MrBayes commands directly executable in PAUP * and MrBayes, or without any specific commands. If favoured, FASconCAT dispenses output files in PHYLIP format with relaxed (unlimited signs) or restricted taxon names (up to ten signs) while sequences are printed in non-interleaved format. FASconCAT is implemented in Perl and freely available from http://software.zfmk.de. It runs on UNIX and MS Windows operating systems.