The purpose of this study is twofold: (1) to examine teacher trainees' knowledge of teaching prior to training and its relationship to performance in the training program; and (2) to determine if there is a set of identifiable domain-specific knowledge related to course performance. Since trainees enter teacher training programs with prior knowledge and beliefs about teaching and learning, their learning and practice will be influenced by such pre-existing knowledge. This paper reports an initial attempt to explore prior domain-specific knowledge of high and low performance trainees. It was hypothesized that trainees' preexisting knowledge in teaching helps performance in course work. Approximately 300 trainees in a secondary school program in Singapore identified one positive and one negative critical incident after viewing a videotaped lesson. Findings suggest: high performance trainees are more critical than low performance trainees with respect to teachers' planning; lower performance trainees are more impressed by teaching aids, interaction, and communication in the classroom; relationships of trainees' comments on the videotaped lesson as a measure of preexisting domain-specific knowledge and their training performance provide insights into the prediction of teaching performance and the selection of trainees. (LL)