Characteristics of thermophilic bacteria and secondary materials on the pyrrhotite surface were investigated by using scanning electron microscopy (SEM). The thermophilic bacteria from an acid hot spring in Japan were incubated with pyrrhotite at $42^{circ}C$, $52^{circ}C$, and $62^{circ}C$ respectively. SEM analysis of the reacted pyrrhotite showed that indigenous rod-shaped bacteria ranging from $0.4{ imes}1.5{mu}m$ to $0.3{ imes}11.9{mu}m$ in size were attached to the pyrrhotite surface at these temperatures with formation of secondary materials. Extracellular polymer substances were formed on the bacterial surface. We suggest that these polymers functioned as a capsule protecting bacteria from the extreme environment. Secondary materials such as elemental sulfur, Fe-hydroxide, S-Fe and O-P-Fe compounds were found on the pyrrhotite surface.