The caryophyllidean tapeworm Atractolytocestus huronensis, which was first detected in Hungary in the intestine of pond-farmed common carp (Cyprinus carpio) in 2001, has rapidly spread throughout the country and is now present in the common carp stock of several fish farms. This parasite has also been detected in market-size common carp imported from the Czech Republic. The cestode can infect young fry of a few weeks of age and older age groups alike. A. huronensis specimens measuring 0.7-1.5 cm characteristically colonise the proximal segment of the common carp intestine containing also intestinal crypts, where they are permanently attached to the intestinal wall by forcing their muscular, spear-shaped head into the gut mucosa and then changing it into a widening cone shape. The scolex, which presumably makes its way into the mucosa through an intestinal crypt, causes atrophy and disruption of the intestinal epithelium; as a result, it will be separated from the lamina propria of the mucous membrane by the basement membrane only. The basement membrane surrounds the scolex, and only a few islets of degenerated epithelium can be seen between the worm and the membrane. The gut epithelium coming into contact with the strobila of the tapeworm is flattened and the cytoplasm of epithelial cells is degenerated. In the affected areas, large numbers of cell nuclei not surrounded by cytoplasm can be seen. Some of these nuclei exhibit karyorrhexis but inflammatory changes cannot be detected. Tapeworms in the gut lumen are surrounded by numerous cellular elements including tissue cells with damaged cytoplasm, red blood cells, lymphocytes and macrophages; however, eosinophilia usually seen in cestode infections cannot be demonstrated. Up to this time no losses due to Atractolytocestus infection were recorded.