College instructors in the United States usually make their own decisions about how to design course exams. Even though summative course exams are well known to be important to student success, we know little about the decision making of instructors when designing course exams. To probe how instructors design exams for introductory biology, we conducted an exploratory interview study with seven instructors teaching the same introductory biology course at a large university. We found that despite designing exams for the same course, instructor exam decisions differed with regard to what content was assessed, the exam format, the cognitive difficulty of exam questions, the resources used when crafting exams, and how exams were administered. We hope that this work can initiate conversations about how college instructors should design exams and lead to more uniformity in how student learning is assessed across the same courses taught by different instructors.