Resistant (65-410 and 65-393) and susceptible (breeders Lahontan) alfalfa seedlings were infected with the Northern root-knot nematode, Meloidogyne hapla. Infected roots were fixed, stained, and embedded in plastic for ultrastructural investigations. Sections of plastic embedded tissue, cut at 0.5-1.5 microns for light microscopy investigations, were stained with various histochemical stains to determine the chemical composition of the nematode-altered tissues. In the susceptible cultivar, giant cells were formed by enlargement of nematode-stimulated cells without cell wall lysis. The giant cells became multinucleate by karyonkinesis without cytokinesis. Proplastids in the giant cells became structurally altered to form tubular complexes. The resistant plants usually responded to the nematodes by a hypersensitivity reaction followed by extensive wall buildup in cells surrounding the infection sites. Nematodes often remained in the necrotic tissues.