Anxiety is the top mental health concern for undergraduates. While researchers have identified ways that in-person science courses can affect anxiety, little is known about how online science courses affect anxiety. In this study, 2111 undergraduates at a large research-intensive institution completed survey questions about their anxiety in large-enrollment online science courses. Specifically, we assessed students' anxiety in the context of online science courses and asked what aspects of online science courses increase and decrease their anxiety. Students also identified what instructors can do to lessen anxiety in online classrooms. We used open coding and logistic regression to analyze student responses. More than 50% of students reported at least moderate anxiety in the context of online college science courses. Students commonly reported that the potential for personal technology issues (69.8%) and proctored exams (68.0%) increased their anxiety, while being able to access content at a later time (79.0%) and attending class from where they want (74.2%) decreased their anxiety. The most common ways that students suggested that instructors could decrease student anxiety is to increase test-taking flexibility (25.0%) and be understanding (23.1%). This study provides insight into how instructors can create more inclusive online learning environments for students with anxiety.