The lack of agreement among previous measurements of differential intensity sensitivity indicates that the values obtained depend to a large extent on the experimental conditions. The relative importance of various factors is indicated, and a procedure is suggested which was designed to give the smallest possible values of differential intensity sensitivity. Intensive measurements made by this method upon a single subject, using a pure tone of 1000 cycles gave values consistently smaller than any previously reported. There is no sharp division between intensity changes which can be perceived and those which cannot. The response of the subject is essentially variable and can only be described by statistical methods.