  We present observations of the 22 GHz water vapor megamasers in the Circinus
galaxy made with the Tidbinbilla 70m telescope. These observations confirm the
rapid variability seen earlier by Greenhill et al (1997). We show that this
rapid variability can be explained by interstellar scintillation, based on what
is now known of the interstellar scintillation seen in a significant number of
flat spectrum AGN. The observed variability cannot be fully described by a
simple model of either weak or diffractive scintillation.
