A study was carried out on the occurrence and activity of chitinolytic planktonic bacteria of Aeromonas sp., Aeromonas hydrophila and Aeromonas salmonicida species, isolated from lake Jeziorak. Among the identified strains decomposing chitin the most abundant were Aeromonas sp.. All the investigated strains showed maximum chitinolytic activity at pH 6.0. An increase in chitinolytic activity was observed that occurred along with temperature growth (10 degrees - 40 degreesC) and colloidal chitin concentration in the medium (D.5 - 2.5%). Their decomposing activity was most intense after a 192 h incubation time. No strain displayed activity after a 48 h incubation time.