Argon-40/argon-39 age spectra on anorthoclase phenocrysts from three pumice clasts in the KBS Tuff yield nearly ideal flat patterns, providing good evidence that the samples have remained undisturbed since crystallization. The ages are concordant at 1.88 ± 0.02 million years and confirm that the KBS Tuff, a key marker bed in the Koobi Fora Formation, northern Kenya, is now very well dated. These results resolve the conflict between earlier argon-40/ argon-39 and conventional potassium-argon dating measurements on the KBS Tuff. -- AATA