Microbial aggregates of an aerobic granular sludge can be used for the treatment of industrial or municipal wastewater, but their formation from a microbial activated sludge requires several weeks. Therefore, the aim of this research was the selection of microbial cultures to shorten the granule-forming period from several weeks to a few days. An enrichment culture with the ability to accelerate granulation was obtained by repeating the selection and batch cultivation of fast-settling microbial aggregates isolated from the aerobic granular sludge. Bacterial cultures of Klebsiella pneumoniae strain B and Pseudomonas veronii strain F, with self-aggregation indexes of 65 and 51%, respectively, and a coaggregation index of 58%, were isolated from the enrichment culture. A mixture of these strains with the activated sludge was used as an inoculum in an experimental sequencing batch reactor to start up an aerobic granulation process. Aerobic granules with a mean diameter of 446 +/- 76 mu m were formed in an experiment after 8 days of cultivation, but microbial granules were absent in controls. Considering biosafety issues, K. pneumoniae strain B was excluded from further studies, but P. veronii strain F was selected for larger-scale testing.