Propagation of sediment ripples behind mounds of sediment in uniform flows of water over flat beds of fine sand was studied in a recirculating open channel. If the flow is strong enough, scour downstream of the mound, due to flow separation and impingement of strong eddies on the bed, generates a ripple, which itself grows large enough to cause downstream scour and production of a second ripple; the result is a widening train of ripples. Propagation vs. non-propagation, as a function of mound height and bed shear stress, is interpreted in terms of the tentative hypothesis that ripple development is governed by the relationship between minimum height of bed irregularity necessary to generate ripples on an otherwise flat bed and the maximum height of bed irregularity that can be built up by flow over an originally flat bed. Minimum mound height for propagation of ripples was found to be non-zero even when there was some sediment movement on the surrounding flat bed, but there was no sign of spontaneous development of ripples on the flat bed under these conditions; this suggests that there is a range of bed shear stresses for which a flat bed in uniform flow is metastable rather than unstable.