We developed a rubric for measuring students' ability to reason and write scientifically. The Rubric for Science Writing (Rubric) was tested in a variety of undergraduate biology laboratory courses (total n = 142 laboratory reports) using science graduate students (teaching assistants) as raters. Generalisability analysis indicates that the Rubric provides a reliable measure of students' abilities (g = 0.85) in these conditions. Comparison of student performance in various biology classes indicated that some scientific skills are more challenging for students to develop than others and identified a number of previously unappreciated gaps in the curriculum. Our findings suggest that use of the Rubric provides three major benefits in higher education: (1) to increase substance and consistency of grading within a course, particularly those staffed by multiple instructors or graduate teaching assistants; (2) to assess student achievement of scientific reasoning and writing skills; and (3) when used in multiple courses, to highlight gaps in alignment among course assignments and provide a common metric for assessing to what extent the curriculum is achieving programmatic goals. Lastly, biology graduate students reported that use of the Rubric facilitated their teaching and recommended that training on the Rubric be provided to all teaching assistants. (Contains 2 figures and 10 tables.)