Clay minerals of several hydrothermally altered zones in a 1200-m biotite-granite core from a drillhole in northern Switzerland were studied microscopically, by XRD and by electron microprobe. The minerals principally affected by the hydrothermal alteration were plagioclase (An5-An20) and, to a lesser extent, biotite. Illite, regularly interstratified illite-smectite and dioctahedral chlorite-smectite, dioctahedral chlorite, trioctahedral chlorite and kaolinite were detected in the alteration products. Commonly, two or more clay minerals occurred together in pseudomorphs after plagioclase. The mineral chemistry of the clay minerals showed a predominance of the substitution KAl for Si and, to a lesser extent, MgSi for Al. Fluid-inclusion data and the absence of pure smectite and epidote indicated temperatures of ∼200°C for the fluid that caused this alteration.