Some of the most common complaints for which the pet fish practitioner is called upon to examine and diagnose are cutaneous lesions in koi. The majority of the time these lesions will be cutaneous ulcers. Along with “mouth rot’ and “fin rot”, these form the complex described as koi ulcer disease. Treatment protocols will vary depending on whether the lesions are of the trunk, the mouth or the fins or in combination. It will also be dependent on the extent of the lesion(s), the stage of degeneration/regeneration and the treatment options available. It is important to determine the stage in the development and healing as local debridement will be useful in the degenerative phase, but counterproductive during healing. This would limit the use of topical treatment to only the first few days after ulceration. Determination of stressors in the aquasystem leading up to the outbreak should be identified where present. Systemic antibiotics are often indicated, especially with multiple and/or extensive lesions. Additional considerations are maintenance of ideal water quality, especially if treating in a quarantine system and stabilized water temperature at 75°F (24°C), the preferred optimum temperature for koi. Other conditions that may be seen on the integument are saprolegniasis, koi herpesvirus (CyHV-3), edema (“pine cone disease”), macroscopic parasites (Lernaea, Argulus, ich), neoplasia (carp pox, papilloma, squamous cell carcinoma), trauma (heron attack), special conditions of butterfly koi fins, special conditions of Doitsu koi, and finally, clinically significant non-lesions (narial folds of showa, narial folds of butterfly koi, shimmies).