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 \mathrm { 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 100 \mathrm { GeV } is 8.3 \mathrm { \% } of the Crab Nebula flux , and the energy spectrum is reproduced by a power law of photon index 3.64 \pm 0.39 \mathrm { ( stat . ) } \pm 0.25 \mathrm { ( 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 .