This study assessed 1,706 eighth-grade students' reasoning abilities through a critique task in an integrated argumentative reading and writing test. Results indicate that the majority of students did not detect fallacious arguments or clearly explain problems in the arguments. They encountered various challenges in critiquing an argument: (a) being off-task, (b) failing to identify fallacious arguments, (c) having difficulty explaining specific problems in reasoning, and (d) not connecting their criticisms with particular parts of the text being critiqued. Task design and instructional implications are discussed.