Several earthquake sequences have occurred at Mount St. Helens as clusters of shallow events with nearly identical waveforms (multiplets). We present a technique that allows the relative relocation of events within a multiplet to at least an order of magnitude higher precision than is typically possible with traditional techniques. A multiplet is studied in pairs of events called doublets. Each doublet is made up of a reference event and one other event from the multiplet. The cross spectrum of a short window beginning just before the P arrival is used to compute differences in arrival times between seismograms from the same station for each doublet. These time differences, computed with a precision of 1--2 ms, are then used to locate each earthquake relative to the reference event. Engineering explosions with wellknown locations are used to test the relocation technique. Explosions can be relocated with accuracy better than 20 m for events within 250 m of the reference explosion. The relocation technique is applied to earthquakes preceding the September 10, 1984, and the May--June 1985 dome-building eruptions of Mount St. Helens. Forty events that occurred during a 12-hour period in September 1984 are located in a volume, approximately 30 m in diameter, beneath the lava dome. The earthquakes following the extrusion do not occur in multiplets and appear to be distributed over a much larger volume. In the May--June 1985 eruption we recognize three multiplet sets that took place successively in time. The multiplets are interpreted as repeated failure or slip within a relatively small volume due to high strain rates around the magma supply conduit. ¿American Geophysical Union 1987