This paper examines the life history and demography of 3 populations of the polychaete Ophryotrocha japonica, an alien species that until 2001 had only been reported in harbour environments along the Asian and American coasts of the Pacific. The populations analysed came from the Italian harbours of Leghorn and Ravenna, and from the brackish Mar Piccolo of Taranto. The 3 strains are inter-fertile, both with one another and with their Pacific counterparts, but were nevertheless found to have morphological, reproductive and demographic differences. In the Leghorn and Taranto populations there is a sort of sexual dimorphism based on body size that is not present in the Ravenna strain, while the most important differences concern their reproductive characteristics, with great variations being observed in their respective fecundity, egg size and reproductive investment. The differences in life history have very important consequences for the demography and fitness of each population. In particular, morphometric, reproductive and demographic characteristics set the Leghorn population apart from the other 2; it reaches sexual maturity more rapidly, lives longer, is decidedly larger and its population growth rate lambda is 2.26 wk(-1). The Ravenna and Taranto populations appear to be more alike; their eggs are the same size, they are equally fecund, make a similar reproductive investment and have lambda-values of 1.82 and 1.83 wk(-1), respectively. The marked differences observed among the 3 populations may depend on the species' adaptation to different environmental situations, but could also be the result of independent introductions from different localities or at different times.