An examination was made of Giemsa-stained microfilariae in thin blood films from (n = 9) dogs naturally or experimentally infected with Dirofilaria immitis or Dirofilaria repens. Morphological measurements (total length, length of cephalic space, anterior end to nerve ring and last body nucleus, and nucleus-free tail tip) were made on 2–6 microfilariae from each dog with the use of digitally captured images and imaging software. The microfilariae of D. repens were significantly greater (P < 0.001) in all measured dimensions except for the length of the cephalic space, which was significantly shorter (P < 0.001) than that of D. immitis. The cephalic space of D. repens was characterized by being short and routinely being terminated by a distinct pair of nuclei that were separate from the remaining somatic nuclei of the microfilaria. The cephalic space of the smaller microfilaria of D. immitis was longer and did not have the distinct nuclei separated from the somatic column nuclei near the anterior end. The character of the cephalic space seems to be a criterion that could be routinely used for the easy differentiation of these 2 microfilariae in stained blood films.