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PG 1553+11 was the target of a coordinated three week multiwavelengthcampaign during 2003 April and May. A significant X-ray flare was observedduring the second half of this campaign. Although no optical flare was recordedduring the X-ray campaign, optical observations obtained immediately prior tothe campaign displayed a higher flux than that recorded during the campaign. Anoptical flare was observed a few days after the end of the X-ray campaign andmay be related to the X-ray flare. Radio observations were made at threefrequencies, with no significant changes in flux detected near the times of theoptical and X-ray flares. The spectral energy distributions and flux ratios indifferent wavebands observed for this object are compared to other X-rayselected blazars to demonstrate how PG 1553+11 is an extreme member of thisgroup.Comment: 28 pages, 4 figures, 7 tables. Accepted for publication in A

Multiwavelength Observations of the Extreme X-Ray-selected BL Lacertae Object PG 1553+11 (1ES 1553+113)