Context. There is considerable interest in the possible interaction between parent stars and giant planetary companions in 51 Peg-type systems. Aims. We shall demonstrate from MOST satellite photometry and Ca II K line emission that there has been a persistent, variable region on the surface of τ Boo A, which tracked its giant planetary companion for some 440 planetary revolutions and lies ~68° ( $\phi=0.8$) in advance of the sub-planetary point. Methods. The light curves are folded on a range of periods centered on the planetary orbital period, and phase-dependent variability is quantified by Fourier methods and by the mean absolute deviation (MAD) of the folded data for both the photometry and the Ca II K line reversals. Results. The region varies in brightness on the time scale of a rotation by ~1 mmag. In 2004 it resembled a dark spot of variable depth, while in 2005 it varied between bright and dark. The 2004 light curve gives a spot rotation period of $3.5 \pm0.7$ d compared to the known planetary orbital period of 3.3125 d. The amplitude spectrum of the 2005 light curve shows no marked peak at the orbital period but the mean absolute deviation (MAD) of the light curve has a well-defined maximum (half width ~0.15 d) centered on the orbital frequency. Over the 123 planetary orbits spanned by the photometry, the variable region detected in 2004 and in 2005 are synchronized to the planetary orbital period within 0.0015 d. The Ca II K line in 2001, 2002, and 2003 also shows enhanced K-line variability centered on $\phi=0.8$, extending coverage to some 440 planetary revolutions. Conclusions. The apparently constant rotation period of the variable region and its rapid variation make an explanation in terms of conventional star spots unlikely. The lack of complementary variability at $\phi=0.3$ and the detection of the variable region so far in advance of the sub-planetary point excludes tidal excitation, but the combined photometric and Ca II K line reversal results make a good case for an active region induced magnetically on the surface of τ Boo A by its planetary companion.
