Rudd (Scardinius erythrophthalmus) is a cyprinid fish native to Europe that was illegally introduced into New Zealand in 1967. Between the 1960s and 1980s rudd were illegally spread to a number of lakes, ponds, and rivers in New Zealand, principally from the Waikato north. They now also occur in the Wanganui, Manawatu, Nelson, and Canterbury regions. Rudd undergo ontogenetic changes in diet as they grow. Young-of-the-year rudd (58–65 mm mean fork length (FL)) ate a mixture of planktonic cladocerans and chironomid pupae, and potentially competed for these foods with common smelt (Retropinna retropinna). Larger rudd (100–149 mm FL) were primarily benthivorous, and potentially competed with perch (Perca fluviatilis) of the same size, brown bullhead catfish (Ameiurus nebulosus), and probably common bullies (Gobiomorphus cotidianus). Rudd of still larger sizes were increasingly herbivorous, until at >200 mm FL their diet was >80% plant material. As rudd prefer native species of aquatic macrophytes to the introduced species, they can probably modify native plant communities and aid the invasion of introduced aquatic weeds. They may also have contributed to the switch of Hamilton Lake from a macrophyte-dominated state to a phytoplankton-dominated state. It is time for the threats posed by rudd to be recognised, and for an education campaign to be mounted. As past rudd introductions have been done outside the law, increasing the severity of penalties for further illegal transfers is unlikely to be effective, and the coarse angling community should instead be included in management decisions concerning rudd.