Topmouth gudgeon, Pseudorasbora parva (Temminck and Schlegel), is a small cyprinid (maximum recorded: 110mm fork length; Cakic, Lenhardt, Kolarevic, Mikovic & Hegedis 2004) native to Japan, China, Korea and the River Amur basin. Its life history traits, including early maturity (sexually mature at 1 year), batch spawning, nest guarding and broad environmental tolerance limits, favour its ability to succeed in invading new water bodies (Ricardi & Rasmussen 1998) and have undoubtedly assisted the rapid invasion of P. parva in European water bodies (Gozlan, St-Hilaire, Feist, Martin & Kent 2005). Consequently, P. parva has been classified as an international pest species (Welcomme 1992). Until recently, the hypothesised risks that this Asiatic invader poses to European freshwater biodiversity, such as out-competing other fishes for food resources and preying on the eggs and larvae of other species (Stein & Herl 1986; Xie, Cui & Li 2001), have remained unquantified (Pinder 2005). However, the recent discovery that P. parva is a healthy host to an emergent infectious disease (Rosette-like Agent, closely related to Sphaerothecum destruens), which threatens European fish diversity (Gozlan et al. 2005), is the first real evidence that P. parva is capable of causing real ecological damage to freshwater ecosystems outside its native range.

This note reports on the rapid colonisation of new water bodies by P. parva in England and Wales and provides its current known distribution. Given the severe impacts that P. parva could have on native populations, this provides an important basis for those agencies responsible for protecting aquatic ecosystems in the UK, as it highlights previously unidentified P. parva populations. It also identifies those sites from which there is an imminent risk of downstream dispersal of P. parva into river networks, an occurrence that would greatly reduce options for controlling the future spread and subsequent impact of this alien species. Following the rapid invasion of mainland Europe (Arnold 1985, 1990; Bianco 1988; Wildekamp, Van Neer, Kucuk & Unlusayin 1997; Caiola & Sostoa 2002; Gozlan, Pinder & Shelley 2002; Pinder & Gozlan 2003), it was no surprise when P. parva was recorded in England in 1996, following its discovery in a pond in Southern England (Domaniewski & Wheeler 1996). Although the source of this population is not known, Gozlan et al.(2002) indicated that the earliest and only known introduction to the UK occurred more than a decade earlier, when an ornamental aquaculture facil-