This study investigated the relationship between receptive and productive vocabulary size. The experimental design expanded upon earlier methodologies by using equivalent receptive and productive test formats with different receptive and productive target words to provide more accurate results. Translation tests were scored at two levels of sensitivity to measure receptive and productive knowledge of meaning and form. The results showed that total receptive vocabulary size was larger than productive vocabulary. When responses were scored for fuller knowledge, receptive vocabulary size was also found to be greater than productive vocabulary size in each of three word frequency bands, with the difference between receptive and productive knowledge increasing as the frequency of the words decreased. However, when responses were scored for partial knowledge, there was little difference among vocabulary sizes at each frequency band. The findings also indicated that receptive vocabulary size might give some indication of productive vocabulary size. Learners who have a larger receptive vocabulary are likely to know more of those words productively than learners who have a smaller receptive vocabulary. (Contains 4 tables and 2 figures.)