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The BL Lacertae object Markarian 501 has shown very high activity
from March to October in 1997. During this period the gamma ray
emission was detected with ground based observations by the Utah
Seven Telescope Array (USTA), Whipple, HEGRA, and CAT in the TeV
energy range. We find evidence for periodicity of 23.9 days from
TeV gamma ray light curves using the USTA data, and about 24 days
using other group's data. This periodicity may be related with the
dynamics of the central engine of the AGN, the rotation of the
central black hole(s) or accretion disk, or the precession of a jet.
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