This study focused on informal reasoning regarding socioscientific issues. It sought to explore how content knowledge influenced the negotiation and resolution of contentious and complex scenarios based on genetic engineering. Two hundred and sixty-nine students drawn from undergraduate natural science and nonnatural science courses completed a quantitative test of genetics concepts. Two subsets (n = 15 for each group) of the original sample representing divergent levels of content knowledge participated in individual interviews, during which they articulated positions, rationales, counterpositions, and rebuttals in response to three gene therapy scenarios and three cloning scenarios. A mixed-methods approach was used to examine the effects of content knowledge on the use of informal reasoning patterns and the quality of informal reasoning. Participants from both groups employed the same general patterns of informal reasoning. Data did indicate that differences in content knowledge were related to variations in informal reasoning quality. Participants, with more advanced understandings of genetics, demonstrated fewer instances of reasoning flaws, as defined by a priori criteria, and were more likely to incorporate content knowledge in their reasoning patterns than participants with more naïve understandings of genetics. Implications for instruction and future research are discussed. © 2004 Wiley Periodicals, Inc. Sci Ed89:71–93, 2005