  We present the results of five years (2005-2009) of MAGIC observations of the
BL Lac object PG 1553+113 at very high energies (VHEs, E > 100 GeV). Power law
fits of the individual years are compatible with a steady mean photon index
\Gamma = 4.27 $\pm$ 0.14. In the last three years of data, the flux level above
150 GeV shows a clear variability (probability of constant flux < 0.001%). The
flux variations are modest, lying in the range from 4% to 11% of the Crab
Nebula flux. Simultaneous optical data also show only modest variability that
seems to be correlated with VHE gamma ray variability. We also performed a
temporal analysis of (all available) simultaneous Fermi/LAT data of PG 1553+113
above 1 GeV, which reveals hints of variability in the 2008-2009 sample.
Finally, we present a combination of the mean spectrum measured at very high
energies with archival data available for other wavelengths. The mean spectral
energy distribution can be modeled with a one-zone Synchrotron Self Compton
(SSC) model, which gives the main physical parameters governing the VHE
emission in the blazar jet.
