The present paper describes the structure of MmtDB-a specialized database designed to collect Metazoa mitochondrial DNA variants. Priority in the data collection is given to the Metazoa species for which a large amount of variants is available, as it is the case for human variants. Starting from the sequences available in the Nucleotide Sequence Databases, the redundant sequences are removed and new sequences from other sources are added. Value-added information are associated to each variant sequence, e.g. analysed region, experimental method, tissue and cell lines, population data, sex, age, family code and information about the variation events (nucleotide position, involved gene, restriction site's gain or loss). Cross-references are introduced to the EMBL Data Library, as well as an internal cross-referencing among MmtDB entries according to their tissual, heteroplasmic, familiar and aplotypical correlation. MmtDB can be accessed through the World Wide Web at URL [see text].