In oil recovery operations, the application of xanthan biopolymers is considered more environmentally friendly than that of synthetic polymers. However, a decrease in the viscosity of xanthan has sometimes been observed. The objective of the present study was to investigate the decrease in the viscosity of xanthan gum caused by bacterial activity in samples from fracturing waters in the Golfo San Jorge Basin, Argentina. To this end, solutions of xanthan gum dissolved in culture medium were studied by means of viscometry and Fourier Transform infrared spectroscopy. The intrinsic viscosity of xanthan in the original sample decreased in 48-72 h, whereas that in two bacterial communities grown in a previous culture medium was not modified.