A 39-yr-old female beluga (Delphinapterus leucas) presented with lassitude and slightly labored respirations. A presumptive diagnosis of pneumonia was made based on ultrasound findings and blood analyses. Initial therapy included broad-spectrum antibiotics and antifungal medication. Ten days later, the whale developed a prominent 8 x 10 cm subcutaneous swelling craniodorsal to the left eye. A fine-needle aspirate revealed a purulent grey-tan exudate that was submitted for bacterial and fungal culture. Cytology revealed numerous degenerative polymorphonuclear leukocytes (PMNs), however no organisms were seen. The abscess was drained using a 1.2 cm diameter stainless steel punch followed by the placement of a Seton drain. Within three days, a culture grew a pure colony of a Nocardia species (later identified as N. asteroides complex by Microbiology Specialists, INC., Houston, TX 77054). Drug sensitivities to this organism prompted a change in antibiotic therapy to a Trimethoprim sulfa antibiotic at a dose of 11 mg/kg once daily. Trimethoprim sulfadiazine (TMS) at a ratio of 1:2 (normal formulation is 1:5) was chosen based on in vitro and in vivo laboratory analyses of pharmacodynamics using a murine model to evaluate the pathogenicity of past Nocardia species isolates. During the initial 75 days of therapy the animal's behavior and appetite remained normal. Approximately two months after therapy was discontinued, subtle behavior changes prompted a follow-up blood analysis which demonstrated an elevated white blood cell count with a moderate left shift. Considering the possibility of disease recrudescence, even in the absence of cutaneous signs, TMS therapy was resumed for an additional 45 days. The only clinical pathology abnormalities observed were a transient mild thrombocytopenia and mild azotemia. After withdrawing the drug, the platelet count returned to normal within 7-14 days and the azotemia improved after reducing the TMS dose by 25%. The whale died of unrelated causes approximately one year following the initial diagnosis. Post-mortem examination showed no evidence of nocardiosis.