This study was intended to determine whether or not the genre of a reading text affects the incidental vocabulary acquisition of L2 learners while reading. To this aim, 40 Iranian EFL students whose vocabulary knowledge was within a limited range (already determined by Nation's Vocabulary Levels Test) were divided into two groups of 20 each for the reading sections. The Narrative Group comprised the participants who read the narratives, and the Expository Group were those who read the expository texts. Three types of vocabulary tests (i.e., Form recognition, Meaning translation and Multiple-choice items) were administered after the reading sessions to assess the incidental vocabulary gains of the participants. Overall, this study demonstrated the relative superiority of expository texts over narratives in terms of enhancing readers' incidental acquisition of unknown words. It is argued that depending on the genre of a text, readers will invest processing resources with different depths and varying degrees of cognitive elaboration for the task of comprehension.