This paper addresses the issue of how we might be able to assess productive vocabulary size in second language learners. It discusses some previous attempts to develop measures of this sort, and argues that a fresh approach is needed in order to overcome some persistent problems that dog research in this area. The paper argues that there might be some similarities between assessing productive vocabularies--where many of the words known by learners do not actually appear in the material we can extract them from--and counting animals in the natural environment. If this is so, then there might be a case for adapting the capture-recapture methods developed by ecologists to measure animal populations. The paper reports a preliminary attempt to develop this analogy. (Contains 4 tables.)