When grown in sorbitol-asparagine-salts medium, cells of Erwinia amylovora isolate T83 released a mixture of oligosaccharides and lipopolysaccharide in place of extracellular polysaccharide. The oligosaccharide fraction was usually readily separable from lipopolysaccharide by gel permeation chromatography, but some samples required disaggregation by preliminary treatment with EDTA. By 1H- and 13C-NMR spectroscopy, methylation analysis and mass spectrometry, the oligosaccharide fraction was shown to consist of terminal, beta-1,2- and beta-1,2,6-linked D-glucose residues in the ratio 2.2:2.6:1, indicating a branched oligosaccharide structure. The purified oligosaccharide fraction was partially resolved by gel permeation chromatography into two components containing approximately equal amounts of glucose. When grown on pear slices, isolate T83 produced normal extracellular polysaccharide.