This paper reports on an exploratory case study involving the development of a model for identifying and measuring individual engagement in critical thinking in an online asynchronous discussion (OAD). The model is first developed through a review of the literature on the construct and subsequently applied to the content analysis of the transcripts of eight student participants in an online discussion. The model, which included four critical thinking processes, their descriptions and indicators, proved effective for the identification and measurement of individuals' critical thinking in OADs. Suggestions for further research include additional testing of the model using other raters in other OADs. (Contains 4 tables.)