The effect of white clover mosaic virus infection on several factors relevant to symbiotic N2fixation was determined in nodulated red clover (Trifolium pratenseL.) plants at 10-leaf, preblooming, blooming, and flower-senescence stages of growth, corresponding to 3, 6, 9, and 12 weeks after virus inoculation, respectively. Virus infection caused a significantreduction of plant growth (top and root weight), nodulation (number and weight of nodules), rhizobial population, nitrogenase activity, and leghemoglobin concentration in the nodules. The presence of a high concentration of virus within nodules at all harvest times was indicated by infectivity assays. The leghemoglobin was shown to be localized in the peribacteroidal space by a cytochemical technique. A significant increase in inducible nitrate reductase activity was associated with virus infection of plants.