This study explores the entrepreneurial logic applied in the opportunity evaluation phase in a research-based startup company (RBSO) faced with multiple business opportunities recognized by external actors. Entrepreneurial logic, measured through the enactment of entrepreneurial behavior, is investigated from the theoretical perspectives of causation and effectuation. Making use of a qualitative, single embedded case study consisting of participant observations and interviews conducted during a seven-month period in a Swedish RBSO, this study contributes to research in the field of entrepreneurship by exploring the logic of opportunity evaluation in RBSOs. Our findings show that the combinations of causal and effectual logical patterns are applied in the opportunity evaluation process but that effectual logic prevails. The effectual logic is related to the operational aspect of the evaluation process and is applied internally while causal logic is related to the general strategic aspect and serves as a way to communicate with the company's stakeholders. The primary driver behind the prevalence of the effectual versus causal logic is uncertainty which in turn is related to the knowledge-intensive nature of the RBSO and the dynamic environment in which it operates. The causal logic is mainly found in the company’s acquisition of human resources aimed to support technology-related knowledge gain.