The chemotactic interaction of two thiobacillus spp with sulphur and pyrite provided in the form of spherical particles was studied. Around sulphur globules the concentration of bacteria decreased with 1/r(2) (r distance from centre of globule), indicating that the bacteria responded to sulphur species released from the sulphur globule. This also suggests that these bacteria rely on a chemotactic response. Similar evidence comes from pyrite globules (framboids) around which bacteria generate a ＇halo＇ of submicroscopic sulphide particles. The non-ionic detergent Tween 80, which affects the surface behavior of bacteria, neutralized this phenomenon. It was concluded that interfacial bacterial activity liberates excess sulphur or sulphide carrying species which maintain bacterial life in the surrounding medium (cooperative leaching). This phenomenon which requires chemotactic behavior may have developed as an adaptive advantage. A mathematical model is presented and discussed.