We have characterized a reciprocating mechanical oscillation that can easily be detected in a large fraction of electrofused erythrocytes. Under our conditions, up to about 30% of all electrofusion products (e.g. doublet, triplet, and higher) show at least one cycle and up to 10% show only two cycles. A much smaller fraction (about 1-2%) show 5-10 or more cycles before stopping. In fused doublets the oscillation appears as a roughly linear and slow expansion of the diameter of the 'hourglass constriction' or fusion zone which proceeds simultaneously with a slow contraction of the pole-to-pole length. At what appears to be a threshold, the fusion zone diameter shrinks simultaneously with an expansion in the pole-to-pole length. This takes place rapidly (within a few video frames). The change in length is about 10% and is easily observable by video light microscopy. The periodicity is variable (5-60 s) and subsequent periods often decrease substantially in length. For the range studied, the characteristics of the reciprocation do not appear to be dependent on the strength of the electric field pulse. To our knowledge this phenomenon was originally discovered, but not characterized, in another laboratory. Using our protocol, cells were observed to undergo fusion from as soon as 6, to as late as 84 s after the fusogenic electric pulse was applied.