The refinement of low-grade clay, of which impurities are mainly sulfur and iron compounds, is required because of the recent shortage of high-grade clay for manufacturing of structural ceramics. The major impurity compound contained in the low-grade clay we treated was identified as pyrite by X-ray powder diffraction and inductively coupled plasma analyses. The well-formed crystals of pyrite had a framboidal form of 1 mu m-20 mu m diameter. The microbial removal of pyrite from the low-grade clay was investigated by using a sulfur and iron-oxidizing bacterium, Thiobacillus ferrooxidans. About 82-90% of the pyrite was removed in 5-12 d for pulp densities up to 70% (w/v). The removal rate of pyrite ranged from 270 to 914 mg-pyritic sulfur/l . d depending upon clay pulp density. The rate of pyrite removal (r) could be expressed as a function of pyritic sulfur concentration (S): r (mg-pyritic sulfur/l . h)=1.96 x 10(-2) S (mg-pyritic sulfur/l). The logarithm of the amount of oxidized pyrite per unit volume and the final pH in the reaction medium were found to have a linear relationship which could be expressed as pH=2.43-0.55 log [FeS2 (mM)]. With the refined clay no red color due to the presence of pyrite was developed after firing, and its whiteness was similar to that of a high-grade clay.