We examined undergraduate research experiences (UREs) participants' and their faculty mentors' beliefs about the professional practices and dispositions of research scientists. In Study 1, 63 science interns and their mentors rated Merton's ("J Legal Political Sociol," 1:115-126, 1942) norms and Mitroff's ("Am Sociol Rev," 39 (August):579-595, 1974) counter-norms of scientific practice. Specifically, we investigated what practices they believed research scientists should subscribe to (or not), and what practices they believed actually characterized research scientists' behavior in the real world. Regarding idealized practice, mentors rated the norms significantly higher than did interns; mentors and interns generally did not differ in subscription to the counter-norms. Regarding actual practice, mentors believed scientists' behaviors reflected counter-norms more than norms. Mentors further noted discrepancies between practices that should represent and actually did represent scientists' work. In Study 2, interns and mentors listed characteristics associated with "thinking" and "behaving" like scientists. Personal and professional dispositions were mentioned more than intellectual and research skills. Although there was considerable consensus between faculty and intern perceptions, findings also revealed discrepancies that could be addressed in UREs, thereby aiding undergraduates' socialization into the culture of scientific practice. Suggestions are provided for broadening interns' conceptions of both scientists and science.