Summary This paper describes the principle, design and use of a form of demountable mechanical strain gauge which has been found particularly useful for determining strains in concrete structures. The instrument which uses a lever and dial gauge is cheap to make and use, and is located on 8 in. gauge lengths which are formed by pairs of punched and drilled steel discs glued to the concrete surface. With practice, 200 strains per hour may be measured with an accuracy of about ±5 × 10−6 under most laboratory test conditions. Even greater accuracy has been obtained using the gauge in its ideal horizontal position and with the readings lying within a small range on the dial. In development tests of this kind 90 per cent of measured strains were within ±3 × 10−6 of the mean. Tests on various locating discs show that normal, drilled mild steel discs give the best results.