Two strains of non-flocculating sewage sludge bacteria ( Xanthomonas sp. S53 and Microbacterium esteraromaticum S51) showed 91% and 77% co-aggregation, respectively, with Acinetobacter johnsonii S35 using a spectrophometric assay. The co-aggregates in case of Xanthomonas sp. S53 and A. johnsonii S35 were above 100 mum and stable against EDTA ( 2 mM) and a commercial protease (0.2 mg ml(-1)). Protease/periodate pretreatment of the partners did not affect this co-aggregation. On the other hand, co-aggregates of M. esteraromaticum S51 and A. johnsonii S35 (50 -70 mum) were deflocculated by EDTA or protease. Protease pretreatment of M. esteraromaticum S51 and periodate pretreatment of A. johnsonii S35 prevented their co-aggregation with respective untreated partners. The potential co-aggregation mechanisms of A. johnsonii S35 varied depending upon the other partner involved.