Harmonically related Pc 3--4 pulsations (7--100 mHz) are observed simultaneously by the three geosynchronous satellites ATS 6, SMS 1, and SMS 2, which are separated 20¿ from one another. At a given instance the frequency of the same harmonic is different from one spacecraft to another and each spacecraft observes a decrease in the fundmental frequency as it moves from morning (15 mHz) to afternoon (10 mHz). This frequency behavior is explained in terms of standing alfv¿n waves, for which the frequency is determined by the local magnetic field and plasma density. Occurrence of harmonic Pc 3--4 waves only during daytime hours (0400--2000 LT) and their frequency characteristics suggest a broadband energy source located on the dayside. Possible azimuthal wave number m and azimuthal phase velocity Vϕ of the second through fourth harmonics are determined from an unusual interval during which identical harmonic frequencies were observed at SMS 1 and ATS 6. Under the assumption of tailward propagation of constant--phase fronts at the same velocity for all these harmonics, we obain Vϕ~1700 km/s.