Feedback to the test taker is a defining characteristic of diagnostic language testing (Alderson, 2005). This article reports on a study that investigated how much and in what ways students at a Taiwan university perceived the feedback to be useful on an online multiple-choice diagnostic English grammar test, both in general and by students of higher and lower language proficiency. Stage 1 involved questionnaire data from 68 students who rated each item's feedback according to usefulness, and Stage 2 involved interviews with five students as they read the feedback after taking the test. The data from these two stages showed students' overall positive attitude toward the feedback and students' preferences for particular feedback characteristics. The study also found that although higher proficiency test takers found the feedback to be more useful than lower proficiency test-takers, views about the characteristics of good feedback were similar regardless of level. Recommendations for improving diagnostic language test construction and validation are discussed based upon the findings. (Contains 7 tables, 3 figures and 5 footnotes.)