Fine structural analyses of host tissue (alfalfa, Medicago sativa L.) response to infection by the stem nematode (Ditylenchus dipsaci Kühn) were conducted. Hypocotyl regions were taken on 1,3 and 7 days after inoculation. Electron micrographs of infected tissue indicated the types of damage were the same between Lahontan (tolerant line) and Ranger (susceptible line). Only the infection rate (in percent) and degree of damage were different between lines and among the different temperatures (15, 20 and 25 C). The higher the temperature, the more injury resulted. After 3 to 7 days of infection, the symptoms observed were swelling and broken endoplasmic reticulum (ER), distended and broken chloroplasts, loss of nuclear material and bulging and rupturing of nuclear envelopes. Heavily infected cell walls showed more osmiophilic substances on one side. Infected cytoplasm contained more ER (both rough and smoothER), ribosomes, vesicles and Golgi apparatus, suggesting increased metabolic activities. Lobing nuclei were observed in all treatments. Lipid content varied with temperature in one-day-old seedlings. At 15 and 25 C, electron dense substances were commonly found along the tonplast, intercellular spaces and on the cell wall. Also some enlarged ER were noted in the non-infected controls at these temperatures. From the fine structural studies of host tissue it is not possible at this time to determine the nature of resistance of alfalfa lines to nematode infection. More studies at both the biochemical and electron microscopical levels are needed. Further, studies on the activities of the nematodes at the various temperatures during the infection periods would be primarily important.