  We discovered rapid intra-day variability in radio source PMN J1726+0639 at
GHz frequencies, during a survey to search for such variability with the
Australia Telescope Compact Array. Follow-up observations were conducted over
two years and revealed a clear, repeating annual cycle in the rate, or
characteristic timescale, of variability, showing that the observed variations
can be attributed to scintillations from interstellar plasma inhomogeneities.
The strong annual cycle includes an apparent "standstill" in April and another
in September. We fit kinematic models to the data, allowing for finite
anisotropy in the scintillation pattern. The cycle implies a very high degree
of anisotropy, with an axial ratio of at least 13:1, and the fit is consistent
with a purely one-dimensional scintillation pattern. The position angle of the
anisotropy, and the transverse velocity component are tightly constrained. The
parameters are inconsistent with expectations from a previously proposed model
of scattering associated with plasma filaments radially oriented around hot
stars. We note that evidence for a foreground interstellar cloud causing
anomalous Ca II absorption towards the nearby star Rasalhague ($\alpha$ Oph)
has been previously reported, and we speculate that the interstellar
scintillation of PMN 1726+0639 might be associated with this nearby cloud.
