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Biotic invasions have a reputation for unpredictable behavior. Here, we report how slight changes in human activity responsible for the introduction and range expansion of a non-native mollusk have led to detectable differences in the genetics of the invasion. Cyclope neritea is a non-predatory gastropod introduced to 2 areas of the European Atlantic: the northwest Iberian Peninsula (NWIP) and the French Atlantic coast (FAC). Shellfish seabed farming is intense in both areas but focuses on different commercial species. Using mitochondrial gene sequences, the lower genetic diversity recorded along the NWIP suggests a more homogeneous range of source populations than in the FAC. Unlike FAC, genetic diversity and haplotype composition in the NWIP correlate with the date of first occurrence of C. neritea at each site rather than with geographical location. Although this pattern evokes the genetic signature expected under a serial-founder colonization model from a single initial enclave, a comparison with samples from potential source populations suggests that the NWIP probably experienced several independent reintroductions. The jump dispersal pattern of C. neritea in the NWIP, together with the observation that populations established in the same year are genetically undifferentiated, point to human transport as the most plausible explanation for the current range expansion. Despite evidence for human-mediated dispersal, C. neritea managed to develop a seemingly non-random genetic pattern in the NWIP. It is suggested that caution must be exerted when interpreting genetic patterns in invaders., (© 2012 Wiley Publishing Asia Pty Ltd, ISZS and IOZ/CAS.)