The prevalence of Sphaerothecum destruens, a pathogenic parasite, was studied in two wild populations of topmouth gudgeon (Pseudorasbora parva), an invasive freshwater fish non-native to the Netherlands. Using genetic markers and sequencing of the 18S rRNA gene, we showed the prevalence of this parasite to be 67 to 74%. Phylogenetic analysis demonstrated a high similarity with known sequences of S. destruens. The topmouth gudgeon, which functions as a healthy carrier of the pathogen, is rapidly colonizing the Netherlands, its expansion showing no signs of saturation yet. Both the presence of S. destruens and the rapid dispersal of the topmouth gudgeon are considered to constitute a high risk for native freshwater fish.