Most Indian coals have high ash content of the order of 25-35%. High ash in the coal not only reduces the thermal value of coal but also leads to production of fly ash, which is a major environmental problem. Cleaning with gravity concentration techniques is ineffective and more efficient techniques need to be developed. In recent times, bioflocculation as an alternative preparation method has been reported for a number of mineral systems including high-sulfur coals. In this paper, bioflocculation of high-ash Indian coals has been studied using Paenibacillus polymyxa for two coal samples. A quartz sample was used for comparison purposes. Zeta-potential measurements showed that coal samples and the bacterium were negatively charged over most of the pH range with a point-of-zero-charge (PZC) around pH 2-3 Surface free energy, determined through contact angle measurements, showed that the coal samples were hydrophobic while the bacterium was hydrophilic. Among the coal samples, the coal with the lower ash content exhibited greater hydrophobicity. Adhesion tests revealed that adhesion took place in about 25 min and that maximum adhesion occurred around pH 2. Similarly, flocculation tests showed that the bacterium flocculated coal effectively and efficiently with the best results around pH 2. More than 90% of the coal flocculated in about a minute in the presence of the bacterium while compared to about 20-30% in the absence of the bacterium. Flocculation of quartz was retarded under the same conditions, indicating that it is dispersed. Ash analysis of the flocculated portion showed a decrease in ash by 60% thereby suggesting that selective flocculation of coal is possible. (C) 2002 Elsevier Science B.V. All rights reserved.