The ability of non-flocculating Acinetobacter spp. isolates (A. johnsonii S35 and A. junii S33) to coaggregate with other sludge-constituting bacteria obtained from a sewage sludge sample was assessed by a spectrophotometric assay. A. johnsonii S35 showed significant coaggregation (aggregation index (AI) = 70 to 99%) with 7 strains, Bacillus cereus, Blastomonas natatoria, Mycobacterium sp., Thermomonas brevis, and Enterobacteriaceae family, identified by sequencing of 16S rRNA genes. A. junii S33 coaggregated with 4 strains, which were also coaggregating partners of A. johnsonii S35, as indicated by All values of 59-93%. The microbial adhesion to hydrocarbon (MATH) assay showed that both A johnsonii S35 and A junii S33 possess a hydrophobic character as indicated by their relative cell surface hydrophobicity (RCSH), 88 and 61%, respectively. A good correlation was observed between the degree of coaggregation of the Acinetobacter spp. isolates and the RCSH of their partner strains. The cells of an A. johnsonii S35 spontaneous mutant and A. johnsonii IAM1517, which had a hydrophilic cell surface, did not coaggregate with the other examined isolates. These results demonstrated that hydrophobic interaction plays an important role in the intergeneric coaggregation of Acinetobacter spp. isolates with other sludge-constituting bacteria. (c) 2009, The Society for Biotechnology, Japan. All rights reserved.