Degradation of films made from poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) in brackish Lake Shira at different depths: 3, 9, 13 and 20 m were studied at first. These depths correspond to the stratification levels on temperature, oxygen and some chemical substances. The fastest degradation of films (on the basis of decrease of films weight and molecular mass of polymer) occurred in epilimnion (3 m), in the most warming and aerated level of lake. Films degradation also occurred at 13 m and 20 m where there are low temperatures, hydrogen sulphide and no oxygen.