Abstract in an experiment designed to examine nonresponse bias, either the March of Dimes or the university of Michigan was identified as the sponsor of a survey mailed to individuals whose level of sup-port for the March of Dimes was known. The response rate was higher to the university survey, but support for the March of Dimes increased survey participation to the same extent in both conditions. As a result of the overrepresentation of supporters of the organization, both sur-veys showed nonresponse bias for variables linked to support. The bias was greater, however, when the sponsor was identified as the March of Dimes. Thus, the university sponsor brought in not only more of the Robert M. Groves is Director of the Census Bureau, Washington, DC, uSA. Stanley presser is professor in the Sociology Department and the Joint program in Survey Methodology at the university of Maryland, College park, MD, uSA. Roger Tourangeau is vice president at Westat, Rockville, MD, uSA. Brady T. West is Research Assistant professor at the institute for Social Research, university of Michigan, Ann Arbor, Mi, uSA. Mick p. Couper is Research professor at the institute for Social Research, university of Michigan, Ann Arbor, Mi, uSA. eleanor Singer is Research professor emerita at the institute for Social Research, university of Michigan, Ann Arbor, Mi, uSA. Christopher Toppe is Senior Social Scientist at the Department of State, Washington, DC, uSA. This article is a revision of a paper presented at the 2008 Workshop on household Survey Nonresponse, ljubljana, Slovenia, and the 2009 American Association for public Opinion Research Annual Conference, hollywood, Fl, uSA. The authors are indebted to the uS National Science Foundation for grant support [SeS-0207435 to R.M.G., M.C., e.S., and S.p.]; to the March of Dimes for access to its database; to John lafrance, Courtney kennedy, emilia peytcheva, and haley Gu for assistance on various aspects of the project; and to the anony-mous referees for helpful comments. The views expressed in this article are the authors’ and do not necessarily reflect the views of the organizations with which they are affiliated. *Address correspondence to Stanley presser, university of Maryland, Sociology Department, 2112 Arts-Sociology Building, College park, MD, 20742-1315, uSA; e-mail: spresser@survey.umd.edu.