The present invention relates to viscosified treatment fluids used in well completion and stimulation operations for oil and gas production, and more particularly, to viscosified treatment fluids comprising xanthan gelling agents. In one embodiment, the present invention provides a method of treating a portion of a subterranean formation comprising the steps of: providing a viscosified treatment fluid that comprises water and a gelling agent that comprises a modified xanthan; and treating the portion of the subterranean formation. The present invention also provides methods of fracturing, gravel packing, acidizing with the viscosified treatments fluids. Also provided are viscosified treatment fluid compositions and gelling agent compositions.