NEMBASE4 is a database of nematode transcriptomes assembled from Sanger Expressed Sequence Tag data. Its purpose is to collate in one place data for many species to assist in drug target identification, vaccine candidate searching and other projects. NEMBASE4 incorporates data from 63 nematode species, assemblies of over 600,000 ESTs and over 250,000 predicted proteins. It incorporates extensive annotation and cross-referencing between species, and there is a dedicated BLAST server. All data are freely available (as a SQL database dump, and as nucleotide and protein sequence files).