Aqueous well treatment fluid compositions with biocidal activity are disclosed comprising a polymer for modifying fluid viscosity in the aqueous fluid, an organic monocarboxylic peracid being present in an anti-microbial amount of about 1 ppm to about 1000 ppm, and a controlled amount of hydrogen peroxide also being present. Peracetic acid is the preferred peracid. The viscosity-modifying polymer in the aqueous well treatment fluid composition may serve to reduce friction in the fluid or increase fluid viscosity. A method of using such compositions is also disclosed.
