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We present multiwavelength observations of the BL Lacertae object Markarian501 (Mrk 501) in 1997 between April 8 and April 19. Evidence of correlatedvariability is seen in very high energy (VHE, E > 350 GeV) gamma-rayobservations taken with the Whipple Observatory gamma-ray telescope, data fromthe Oriented Scintillation Spectrometer Experiment of the Compton Gamma-RayObservatory, and quicklook results from the All-Sky Monitor of the Rossi X-rayTiming Explorer while the Energetic Gamma-Ray Experiment Telescope did notdetect Mrk 501. Short term optical correlations are not conclusive but theU-band flux observed with the 1.2m telescope of the Whipple Observatory was 10%higher than in March. The average energy output of Mrk 501 appears to peak inthe 2 keV to 100 keV range suggesting an extension of the synchrotron emissionto at least 100 keV, the highest observed in a blazar and ~100 times higherthan that seen in the other TeV-emitting BL Lac object, Mrk 421. The VHEgamma-ray flux observed during this period is the highest ever detected fromthis object. The VHE gamma-ray energy output is somewhat lower than the 2-100keV range but the variability amplitude is larger. The correlations seen heredo not require relativistic beaming of the emission unless the VHE spectrumextends to >5 TeV.Comment: 10 pages, 2 figures, accepted for publication in ApJ Letter

Multiwavelength Observations of a Flare from Markarian 501