The Chinese mitten crab Eriocheir sinensis is an invasive species that inflicts considerable economical and ecological damage in European and American freshwaters. Sequence variation at the mitochondrial COI gene was investigated from Chinese, European and American populations in order to infer the invasion history. Firstly, we found a decrease in genetic variability from the source populations to those that have recently been established. Secondly, Central European populations that are about 100 yr old seem to have originated from multiple invasion events. Thirdly, the sequence data suggest high levels of gene flow across drainages within continents. Fourthly, the 2 American populations sampled in San Francisco Bay were fixed for a single haplotype. This haplotype was also found in European but not in Chinese populations; this observation provides circumstantial evidence for the unexpected hypothesis that the American populations were founded by a single introduction event from a European source.