The kidneys of dogs chronically infected with Dirofilaria immitis were studied by light, electron, and immunofluorescent microscopy. The glomeruli of dogs with high microfilaremia showed a moderately increased mesangium and thickened glomerular basement membrane. There was a deposition of electron-dense particles which were continuously distributed in the lamina rara interna and lamina densa of the glomerular basement membrane. Immunofluorescent microscopy showed intense IgG deposits in a uniform linear pattern along the glomerular basement membrane. This linear deposition of IgG appeared to correspond to continuous distribution of electron-dense particles in the basement membrane. Microfilariae present in the glomerular capillary lumens were connected with the capillary endothelial cells by narrow cytoplasmic bands. The interaction of the worm's internal substances with the glomerular capillaries through these cytoplasmic bands may deposit worm antigen on the glomerular capillary wall before antigen-antibody interaction takes place. These findings suggest that the glomerulopathy seen in dogs infected with D. immitis is caused by the in situ formation of immune complexes in the glomerular basement membrane.