  We report the first result from our survey of rapid variability in symbiotic
binaries: the discovery of a persistent oscillation at P=1682.6 +- 0.6 s in the
optical emission from the prototype symbiotic, Z Andromedae. The oscillation
was detected on all 8 occasions on which the source was observed over a
timespan of nearly a year, making it the first such persistent periodic pulse
found in a symbiotic binary. The amplitude was typically 2 - 5 mmag, and it was
correlated with the optical brightness during a relatively small outburst of
the system. The most natural explanation is that the oscillation arises from
the rotation of an accreting, magnetic (B_S > 10^5 G) white dwarf. This
discovery constrains the outburst mechanisms, since the oscillation emission
region near the surface of the white dwarf was visible during the outburst.
