The conventional method of setting confidence intervals for the difference of the means of 2 normal populations gives an interval which is not, in general, symmetrical about 0. A modification of the conventional method which leads to symmetry about 0 is discussed and is recommended as particularly appropriate for use in bioequivalence trials. This modification has the effect of decreasing the effective length of the confidence interval, on which the decision concerning bioequivalence is based, while increasing the confidence coefficient.