This study compares the impact of second language (L2) reading and listening on the incidental acquisition and retention of five dimensions of vocabulary knowledge--spoken form, written form, part of speech, syntagmatic association, and form-meaning connection--at the level of recognition (form-meaning connection was measured also at the level of recall). The study also examines the relationship between frequency of word occurrence and vocabulary acquisition through reading versus listening. The participants were 139 pre-intermediate level English as a Foreign Language (EFL) learners with Farsi as their first language (L1), who were assigned to two experimental groups (i.e., reading and listening) and one control group. The experimental groups were exposed to the same text containing 16 target words (replaced by 16 non-words). The results on the immediate posttest revealed that readers scored higher than listeners on all five dimensions of word knowledge. Retention scores on a three-week delayed posttest (which due to the presence of testing effects could be measured for only one dimension of vocabulary knowledge, that is, form-meaning connection) were also higher for readers; however, listeners appeared to forget less within three weeks. Moreover, increase in the frequency of word occurrence in the text significantly benefitted incidental vocabulary acquisition through reading, but not through listening.