This study explored the differences in receptive and productive vocabulary knowledge in terms of word frequency level and vocabulary size in undergraduate learners of English as a Second Language (ESL). A total of 90 first-year undergraduate engineering students from a semi-urban region in India participated in the study. Two quantitative vocabulary tests, the Receptive Vocabulary Levels Test (Schmitt et al., 2001) and the Productive Vocabulary Levels Test (Laufer and Nation, 1999), were applied sequentially to the students. The first test focused on identifying receptive vocabulary size, while the second test measured productive vocabulary size. Data were analyzed using SPSS software. Results indicate that the students' receptive word knowledge is higher than their productive word knowledge. Similarly, the students' receptive vocabulary size is larger than their productive vocabulary size. Furthermore, the difference between their receptive and productive vocabulary size is 27.69%. In order to bridge this gap and increase their vocabulary knowledge and size, we recommend an activity-based, explicit vocabulary teaching approach through self-learning, group learning and mutual learning in the regular classrooms. � 2018 Universidade do Vale do Rio dos Sinos. All rights reserved.