This study examined the relationship between learners' incidental vocabulary acquisition and their level of proficiency, and between acquisition and word frequency in a text. Participants were Turkish learners of English at three proficiency levels. One reading text and four vocabulary tests were administered over a two-week period. Analyses of the data revealed that lexical gains from reading were significant for each group (p < .05). The higher proficiency groups were able to acquire more words than lower level groups. Word frequency in the text was also a significant factor in vocabulary acquisition (p < .05), with 29% of the variance in acquisition being accounted for by frequency. However, frequency did not play a greater role in the vocabulary acquisition of lower level learners than in that of higher level learners.