E. Aliu, S. Archambault, A. Archer, T. Arlen, T. Aune, Anna Barnacka, B. Behera, M. Beilicke, W. Benbow, K. Berger, R. Bird, Markus Böttcher, A. Bouvier, M. Buchovecky, J. H. Buckley, V. Bugaev, J. V. Cardenzana, M. Cerruti, A. Cesarini, Xuhui Chen, L. Ciupik, E. Collins-Hughes, M. P. Connolly, W. Cui, J. Dumm, J. D. Eisch, A. Falcone, Simone Federici, Q. Feng, J. P. Finley, H. Fleischhack, P. Fortin, L. Fortson, A. Furniss, N. Galante, D. Gall, G. H. Gillanders, S. Griffin, S. T. Griffiths, J. Grube, G. Gyuk, M. Hütten, Nils Hakansson, J. Holder, G. Hughes, T. B. Humensky, C. A. Johnson, P. Kaaret, P. Kar, N. Kelley-Hoskins, M. Kertzman, Y. Khassen, D. Kieda, M. Krause, H. Krawczynski, F. Krennrich, M. J. Lang, A. S. Madhavan, G. Maier, S. McArthur, A. McCann, K. Meagher, J. Millis, P. Moriarty, R. Mukherjee, D. Nieto, R. A. Ong, M. Orr, A. N. Otte, D. Pandel, N. Park, V. Pelassa, J. S. Perkins, A. Pichel, Martin Pohl, A. Popkow, J. Quinn, K. Ragan, L. C. Reyes, P. T. Reynolds, E. Roache, J. Rousselle, A. C. Rovero, D. B. Saxon, G. H. Sembroski, K. Shahinyan, F. Sheidaei, C. Skole, A. W. Smith, D. Staszak, Igor O. Telezhinsky, M. Theiling, N. W. Todd, J. V. Tucci, J. Tyler, A. Varlotta, V. V. Vassiliev, S. Vincent, S. P. Wakely, O. M. Weiner, A. Weinstein, R. Welsing, Alina Wilhelm, D. A. Williams, B. Zitzer, M. G. Baring, J. Becerra Gonzalez, A. N. Cillis, D. Horan, D. Paneque

• The very high energy (VHE; E > 100 GeV) blazar Markarian 501 was observed between April 17 and May 5 (MJD 54 938-54 956), 2009, as part of an extensive multiwavelength campaign from radio to VHE. Strong VHE yray activity was detected on May 1st with Whipple and VERITAS, when the flux (E > 400 GeV) increased to 10 times the preflare baseline flux (3.9 x 10(-11) ph cm(-2) s(-1)), reaching five times the flux of the Crab Nebula. This coincided with a decrease in the optical polarization and a rotation of the polarization angle by 15. This VHE flare showed a fast flux variation with an increase of a factor similar to 4 in 25 min, and a falling time of similar to 50 min. We present the observations of the quiescent state previous to the flare and of the high state after the flare, focusing on the flux and spectral variability from Whipple, VERITAS, Fermi-LAT, RXTE, and Swift combined with optical and radio data.