Teacher mathematical noticing is a key component of mathematics teaching expertise and has been a focus of recent professional development efforts. In this paper, I propose and describe explicitly the notion of "productive" mathematical noticing, which surfaces from a case study involving a group of seven mathematics teachers who collaborated as part of a lesson study team at a primary school in Singapore. Two vignettes--one that happened during the planning stage, and the other taken from the actual lesson--are discussed to illustrate the notion of productive mathematical noticing.