The article reports the results of an investigation conducted with a group of Algerian university students majoring in Microbiology. Data for this study came from a reading comprehension test administered to proficient and less-proficient readers. The study addresses the following questions: (i) To what extent do high-level students differ from low-level students as far as vocabulary knowledge and comprehension are concerned? (ii) To what extent do measures of readers' vocabulary knowledge correlate with measures of reading comprehension? As regards the first question, the findings indicated that there were more differences than similarities between the two groups. Concerning the second question, the results of the test yielded an interesting finding indicating that a high performance on one implies also a high performance on the other, and a low performance in one implies a low performance in the other.