  3C 66A is an intermediate-frequency peaked BL Lacertae object of uncertain
redshift. We report recent observations of the region around the blazar with
the MAGIC telescopes. The source was observed and detected in 2009 December and
2010 January, in 2.3 h of good quality data. The signal could clearly be
assigned to the blazar 3C 66A, statistically and systematically rejecting the
nearby radio galaxy 3C 66B as a possible origin of the gamma-ray signal by 3.6
standard deviations. The derived integral flux above 100GeV is 8.3% of the Crab
Nebula flux, and the energy spectrum is reproduced by a power law of photon
index 3.64 \pm 0.39(stat.) \pm 0.25(sys.). Within the errors, this is
compatible with the spectrum derived by VERITAS in 2009. From the spectra
corrected for absorption by the extragalactic background light, we only find
small differences between the four modellings that we applied, and constrain
the redshift of the blazar to z < 0.68.
