Renal neoplasia is described in coypus (Myocastor coypus) from a feral population of the species in East Anglia. A population control campaign was started in 1962, and in 1981 this became an eradication scheme. From 1976 onwards, a research programme included the postmortem examination of 9400 wild caught and captive coypus. During the period 1980-91, 15 cases (0.16%) of renal neoplasia were detected. The tumours were found in both sexes between estimated ages of 25 months and 13 years with no significant sex prevalence. There was no clear evidence that renal tumours were more common in older animals. Tumours were most common in captive coypus and were bilateral in approximately half of the animals. In all cases, the tumours were of epithelial type resembling adenomata and adenocarcinomata of other animals. Most were clearly benign, and, although some showed evidence of malignancy, no unequivocal evidence of metastasis was established. The prevalence of renal tumours in this series is greater than that recorded in previous published surveys of coypus and other rodents. This may relate to the origin of the coypus population, differences in age structure in animals examined, and the varied conditions under which the rodents lived. Aetiological factors remain undetermined.