After high-pressure freezing followed by freeze-substitution of bean nodules infected with Rhizobium leguminosarum biovar phaseoli, thin sections of these nodules exhibit a fibrous component around the bacteroids filling the previously reported empty peribacteroidal spaces. A clear bacterial cell envelope differentiation occurs under symbiotic conditions, The formation of cytoplasmic inclusions is shown to depend mainly on the supplements available to the Rhizobium independently of its living form. (C) 1994 Academic Press, Inc.