Ecologists have long been fascinated by the morphological changes that species frequently undergo when introduced into new regions. In this study an unusual pattern of size change associated with the invasion of 19 species of marine and estuarine invertebrates is reported. The results show that the majority of species are significantly larger in the introduced range compared with the native range with little evidence for any decrease in size following invasion. This invasion-driven increase in body size sharply contrasts with the pattern observed in many other taxa including plants, mammals and lizards, where invaders frequently exhibit post-invasion decreases and increases in size. These size changes were not influenced by differences in latitude, sample size or length of time since invasion. Although several mechanisms, may explain the results, none have been demonstrated.