[en] Measurements of piezoresistance associated with oxygen impurities in Ta are reported. By using an electrical bridge and a phase sensitive detector, a substantial increase in sensitivity is achieved over the pioneering work by Berry and Orehotsky (Berry, B.S. and Orehotsky, J.L., Phil. Mag.; 9:467 (1964)). It is estimated that an oxygen concentration in Ta of 30 atomic parts per million could be detected. (author)