A filler treatment process enhances the fixation of anionic latex on filler in a short time. Anionic polymer dispersions (latex) are added to papermaking filler slurries at ambient temperature and then mixed with water of temperature higher than the glass transition temperature (Tg) of the latex used. The anionic latexes applied by this process are totally and irreversibly fixed or bound onto the filler particles and the treated filler particle agglomerate to form an aggregated filler slurry which is stable over time. The latex-treated filler slurry can be added to papermaking furnishes at any point prior to the headbox of the paper machine or stored for later use. The latex-treated filler slurry improves filler retention, only slightly reduces sheet strength and improves sizing performance.