In summer 2010, a systematic survey was carried out in the harbours of La Spezia (Ligurian Sea) and Olbia (western Tyrrhenian Sea) with the aim of studying alien species in Italian commercial harbours. Biological samples were collected by replicate scraping on the concrete walls of docks at the beginning and at the end of the summer season. Identification to species level revealed the presence of Tricellaria inopinata, an invasive alien cheilostome bryozoan of Pacific origin, first introduced to Europe in the Lagoon of Venice (Italy) in 1982. Thereafter it has been reported in other ports in Britain, Belgium, The Netherlands, France, Spain, Portugal, Tunisia, Ireland, Scotland, Germany and Massachusetts (USA). Its finding in the harbours of La Spezia and Olbia represents the first record of the species in the Ligurian Sea and in the western-central Tyrrhenian Sea, respectively. Commercial harbours are common sites of biotic invasions due to the presence of the most important vectors of alien species introduction, namely aquaculture, and shipping. The occurrence of T. inopinata in the harbours of La Spezia and Olbia is discussed, taking into consideration possible pathways of introduction into the western Mediterranean Sea, which very likely is the transfer of molluscs from the northern Adriatic (namely the Lagoon of Venice).