  PG 1553+11 was the target of a coordinated three week multiwavelength
campaign during 2003 April and May. A significant X-ray flare was observed
during the second half of this campaign. Although no optical flare was recorded
during the X-ray campaign, optical observations obtained immediately prior to
the campaign displayed a higher flux than that recorded during the campaign. An
optical flare was observed a few days after the end of the X-ray campaign and
may be related to the X-ray flare. Radio observations were made at three
frequencies, with no significant changes in flux detected near the times of the
optical and X-ray flares. The spectral energy distributions and flux ratios in
different wavebands observed for this object are compared to other X-ray
selected blazars to demonstrate how PG 1553+11 is an extreme member of this
group.
