Systemic fungal infection in birds are a big challenge in Brazil's wild animal practice, since there is lack of information on adequate avian animal care and also a widely disseminated culture of seed feeding birds, which are kept and sold without any attention to its manipulation and storing. Most of the related cases are of respiratory diseases caused by Aspergillus sp and gastrointestinal pathologies by Candida albicans. The purpose of this work is to document an unusual clinical presentation describing a case of arthritis in a cockatiel, Nymphincus hollandicus, discussing diagnostic techniques, which can contribute to avian practice. The animal, recently bought in a pet shop, was fed on a seed mix basis. Clinical exam revealed apathy, low weight (74 grams), feathers opaque, brittle and with stress lines, soft and greenish stool and, swelling, hotness, and erythema on the right tarsal-metatarsal joint justifying difficult walking. It received a single dose of levamisole (4mg/kg) and was sent home with a prescription for azithromycin 40mg/kg, SID, vitamins (Nutramix®, Mundo Animal, 2 drops/day), and meloxicam 0.2%, (0.5mg/kg, SID). The owners were advised about correct diet, and began the transition from seed to a 50-50 mixture with extruded diet. Seven days later, the animal still presented swelling in both joints and weight loss (body weight = 64 grams). A radiographic exam revealed swelling of the tarsal-metatarsal articulation, compatible with arthritis. To obtain a more accurate diagnosis, material was collected from the joint by fine needle aspiration to prepare smears and, stool was analyzed. Microscopy revealed fungi spores and hyphae in the synovial fluid isolated and inside macrophages as well as in the stool. Prescription was switched to itraconazol 10 mg/kg, SID, PO, with evident recovery. In avian clinic, quick diagnosis is very important, once these animals' anatomy favors the dissemination of the pathogenic agents. Radiographic exam is an auxiliary tool for diagnosis, for it reveals the joint and bone injury degree, but it should not be relied on as the only diagnostic method. Analyses of stool and synovial liquid or a biopsy allow a precise diagnose, since permit the etiologic agent identification. It promotes an accurate, low cost and quick diagnosis, being a very good option for Veterinary Doctor in the avian practice. However, the patients' anamnesis and management, is crucial in providing differential diagnosis, contributing to a successful treatment, recovery and future wellbeing.