Formative computer-based assessments (CBAs) for self-instruction were introduced into a Year-2 field biology module. These CBAs were provided in "tutorial" mode where each question had context-related diagnostic feedback and tutorial pages, and a self-test mode where the same CBA returned only a score. The summative assessments remained unchanged and consisted of an unseen CBA and written reports of field investigations. When compared with the previous three year-cohorts, the mean score for the summative CBA increased after the introduction of formative CBAs, whereas mean scores for written reports did not change. It is suggested that the increase in summative CBA mean score reflects the effectiveness of the formative CBAs in widening the students' knowledge base. Evaluation of all assessments using an Assessment Experience Questionnaire indicated that they satisfied the "11 conditions under which assessment supports student learning". Additionally, evidence is presented that the formative CBAs enhanced self-regulated student learning. (Contains 9 tables.)