Seismograms from WWSSN and Canadian network stations were modeled to determine the source parameters of the 20 June 1978 Thessaloniki, Greece, earthquake (Ms = 6.4). The depth of the initial rupture was constrained to 11 ± 1 km by comparison of the arrival times of surface reflections with synthetic short-period seismograms. A focal sphere plot of first motion polarities provided little constraint on other focal parameters, except to indicate that predominantly normal faulting was involved. A generalized inverse technique utilizing the moment tensor formalism was applied to teleseismic P and SH waves for six increments of depth. The moment tensor obtained indicated a nearly horizontal, N-trending tension axis and a nearly vertical compression axis, and yielded the following double-couple source parameters: strike 280° ± 7°; dip 55° ± 3°; rake −65° ± 5°; seismic moment 5.7 × 1025 dyne-cm; and a skewed triangular source time function with a rise time of about 1 sec and duration of 6 to 8 sec. Due to indications of multiple or finite source effects for this event, and the assumption in the moment tensor formalism of a point source, a low-pass filter was applied to the data and the inversions were repeated. The results were nearly identical with those of the original inversion, suggesting that any individual sources had similar mechanisms, or that the point source model is sufficient for this earthquake.