Between days 10 and 21 after inoculation of Lupinus angustifolius seedlings with Rhizobium NZP 2257, the average nodule fresh weight increased 3-fold and the number of bacteroids per nodule increased more than 10-fold.The viability of Rhizobium bacteroids, as judged by their ability to form colonies on yeast-extract agar, declined from about 10% on days 10 and 11 after inoculation to about 0.3% on days 14 to 25. Bacteroid viability was highly sensitive to osmolarity.At optimal pH and K and Mg ion concentrations, the incorporation of (14)C-glycine into isolated bacteroids was also very sensitive to osmolarity, and fell in parallel with bacteroid viability during nodule development.WE SUGGEST THAT AT LEAST TWO PROCESSES CONTRIBUTE TO BACTEROID NONVIABILITY: a reversible change in the cell wall structure occurring between days 10 and 14 after inoculation, and a subsequent irreversible change.