Over a period of two years four ornamental koi carp (Cyprinus carpio) of one variety in a mixed population of 16 were affected with papillomas of the head and body. In one fish there was a transition of these tumours into a squamous cell carcinoma in the region of the head and posterior gill space, with deep invasion of the underlying bone. One of the fish recovered completely after the sloughing of the papillomas. In view of the progressive nature of the lesions, the condition was presumed to be due to an infectious agent, but transmission electron microscopy failed to reveal any virus particles.
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