The guar polymer and foaming agent-contg. aq. fracturing fluid was modified by addn. of 3,3'-methylene-bis-5-methyloxazoline (I), 2,2-dibromo-3-nitrilopropionamide or glutaral biocides or UV-irradiated to avoid biodegrdn. of the guar. The UV-irradn. was less efficient than the use of biocides. The biocide I was the most efficient.