The sequence of histopathologic changes associated with B. pahangi (Nematoda: Filarioidea) infections in lymphatic vessels in the spermatic cord of the Mongolian jird (gerbil), Meriones unguiculatus, was studied. Intravascular granulomas caused mainly by disintegrating worms were seen in 67% of jirds necropsied on or after 35 days postinoculation [p.i] whereas none of 20 jirds examined before this day showed dying larvae. These granulomas usually evolved without vascular occlusion. Other granulomatous foci, often with a thrombuslike core, sometimes harbored microfilariae or microfilarialike materials. The perilymphatic cellular infiltrate consisted mostly of eosinophils, lymphocytes and plasma cells. Large numbers of eosinophils were seen in the early weeks, but later declined, while lymphocytes increased to become the predominant cell in old infections. Irregular fibrosis of some valves and portions of the lymphatic walls was seen as early as the 2nd wk p.i. Lymphatic changes in the jird are similar to those described in other hosts infected with filariae, but remained moderate. Living worms appeared to be the stimulus for many observed changes. Most pathologic alterations were well establihsed by 3 or 4 mo. and showed little qualitative change during the remaining 4 mo. of the study.