A laboratory study of the exchange of two fluids of different density through a constant-width channel with an underwater sill has enabled us to study Holmboe's instability in greater detail than has been possible in mixing-layer experiments. The internal hydraulics of the exchange flow are such that we have been able to observe the initiation of instability, the development and behaviour of both symmetric and asymmetric Holmboe instabilities, and the suppression of the instability at bulk Richardson numbers above about 0.7. A number of stability criteria resulting from previous numerical investigations have been verified experimentally. Our laboratory measurements are consistent with theoretical predictions of wave speed and wavenumber.