The seeding effect of Pseudomonas aeruginosa, a pathogenic bacterium on ice nucleation in an aqueous environment is of interest because ice formation is always damaging to freeze sensitive plants by the presence of ice nucleating bacteria.,An experiment was conducted at cloud temperature varying from -20degreesC to 0degreesC in a walk-in cold room where lowest attainable temperature is -35degreesC. For a specific total water content seeding was done in three different ways: (i) as a cultured bacterial medium, (ii) by UV-radiation of the cultured bacterial medium for 15 min, and (iii) after boiling the cultured bacteria for 20 min. It was observed that highest amount of nucleation occurred at -3.8degreesC, for cases (i) and (ii). However, when seeding was done by cultured bacteria irradiated by UV-radiation, nucleation reduced remarkably because UV-radiation killed the bacteria leaving aside the residual protein already present. No ice nucleation was noted in the case of seeding by boiled cultured bacteria. Thus we conclude that the residual protein in the outer membrane of the bacterium is mainly responsible for the ice nucleation and hence for crystal formation. (C) 2004 Elsevier Ltd. All rights reserved.