Microliths and ossification were found in the lungs of a 12-year-old bitch suffering from compound mammary gland tumor which has disseminated in the inguinal lymph node glands and the lungs. Pulmonary ossification appeared grossly as irregular stony sharp particles which infiltrated the lung tissue and were readily recognizable from under the pleura as grayish sharp protruding particles. Microscopic examination revealed the existence of intra-alveolar single or multilobular particles of ossification which are formed of lamellated osseous substance with osteocytes in lacunae. Microlith particles were also seen and were smaller, usually solitary and less frequent. They comprised strongly basophilic smooth laminated spherical particles which may enclose faintly stained substances. Both structures were not associated with inflammatory response. Larger particles appeared as white miliary spots by radiography. This article documents for a very rare case of pulmonary microlithiasis and ossification in a dog.