Export and distribution of14C assimilates during a period of steady-state photosynthesis were studied in attached leaflets of the second trifoliate ofPhaseolus vulgarisfor 2 weeks following inoculation withXanthomonas campestrispv.phaseoli. The control was the leaflet opposite the inoculated leaflet at the same node. On either a chlorophyll or leaf area basis, photosynthesis and export declined over time after inoculation compared with control tissue. By 14 days after inoculation, photosynthesis and export were 25 and 50% lower, respectively, in the infected leaflets, even though only 7–10% of the total area measured was necrotic. The reduced synthesis of sucrose was sufficient to account for the reduced export from the infected leaflets. Starch synthesis was reduced to a greater extent in infected leaflets than was sucrose synthesis. In leaf disks infiltrated with mannose or xanthan, an extracelluar bacterial polysaccharide containing mannose, photosynthesis was reduced by 20% and the partitioning of14C photoassimilates into sucrose was reduced but was slightly increased into starch. This is consistent with an effect of xanthan at least in vitro on the triose phosphate/phosphate exchange at the chloroplast envelope.Keywords: photosynthesis, export, xanthan,Phaseolus vulgaris,Xanthomonas campestrispv.phaseoli.