A 10-month-old spayed female Doberman Pinscher was presented for lameness. On physical examination, the dog was lethargic and febrile and had a 2-cm raised subcutaneous mass at the base of the left ear. Fluid from the mass was drained. Direct smears of the fluid, stained with modified Wright's and new methylene blue, were highly cellular and contained large numbers of degenerate neutrophils with moderate numbers of macrophages. Large numbers of round yeast organisms, 8-20 mum in diameter, were observed extracellularly. The organisms had a thick blue wall and granular internal contents and broad-based budding was seen frequently. Branching hyphae or pseudohyphae, with parallel sides and 2-4 mum in diameter, appeared to extend from the surface of the yeast. The morphology of the yeast organisms was consistent with Blastomyces dermatitidis, with atypical hyphae formation. Culture results were not definitive because it was not possible to induce transition from the mycelial to the yeast form at 37 degrees C and because the morphology of the mycelial form of B. dermatitidis could not be differentiated from that of Emmonsia parvae. The organism was confirmed as Ajellomyces dermatitidis (the mycelial form of B. dermatitidis) using 18S ribosome RNA gene sequencing and comparison with an available databank. The mycelial form of B. dermatitidis is rarely found in the tissue of dogs, and may have been induced in this case by low environmental temperatures and the time delay between sample collection and slide preparation.