  We report simultaneous multi-frequency observations of the blazar PG
1553+113, that were carried out in March-April 2008. Optical, X-ray,
high-energy (HE; greater than 100 MeV) gamma-ray, and very-high- energy (VHE;
greater than 100 GeV) gamma-ray data were obtained with the KVA, REM,
RossiXTE/ASM, AGILE and MAGIC telescopes. This is the first simultaneous
broad-band (i.e., HE+VHE) gamma-ray observation of a blazar. The source
spectral energy distribution derived combining these data shows the usual
double-peak shape, and is interpreted in the framework of a
synchrotron-self-Compton model.
