The purpose of this study was to understand what preservice teachers and knowledgeable others professionally notice as they engaged in repeated cycles of a modified version of lesson study, as a component of a field experience in a teacher education program. The study also centered on comparing the professional noticing practices of preservice teachers with other lesson study participants, including classroom teachers and university facilitators. Data analyzed included videos of weekly lesson study analysis meetings for seven weeks for each of four teams. Each team included six preservice teachers, a classroom teacher, and a university facilitator. Findings indicate that preservice teachers primarily noticed elements about the classroom environment and teacher pedagogy, but included instances of noticing centered on students' mathematical thinking. In contrast, classroom teachers and university facilitators, as knowledgeable others, typically noticed general events and were less focused on students' mathematical thinking. Analysis of noticing trends over the seven weeks indicated that noticing levels remained steady initially, dropped in the fourth and fifth week, and resumed original status in the final weeks. Results that the preservice teachers' noticing comments were at higher levels than the knowledgeable others are contrary to other research studies and indicate that incorporating lesson study with appropriate scaffolds into a field experience for preservice teachers may be a viable option for encouraging noticing of students' mathematical thinking.