The source parameters of the Loma Prieta earthquake are determined using long-period Rayleigh waves recorded by USGS/IRIS, IDA/IRIS, and GEOSCOPE stations. The source mechanism is well-constrained by the Rayleigh wave radiatin pattern, with a dip=70(<5¿), strike=130 (¿5¿), rake=135 (¿5¿), and moment=3.4 (¿5¿)¿1019 Nm (MW=7.0). This mechanism is generally consistent with independent body wave determinations. The most stable long-period waves, with periods from 200 to 275 s, indicate that the source process has a centroid time of about 10 s, somewhat longer than that indicated by body waves (about 5--6 s). This discrepancy cannot be uniquely attributed to source effects because of uncertainties in the propagation corrections. The importance of using surface waves with short propagation paths for analysis of moderate size earthquakes such as the Loma Prieta event is demonstrated by the unreasonably long source durations inferred from R3 arrivals. ¿ American Geophysical Union 1990