Most studies which investigate mathematics teacher noticing cast perception into a passive role. This study develops an ecological analysis of mathematics teachers' noticing in order to investigate how teachers actively look for information in classroom environments. This method of analysis is applied to data collected as an experienced primary teacher of mathematics moved between desks and selected students to stop and talk to. An ecological lens draws attention to an active process of looking, employed by the teacher, which locates particular properties in the mathematical representations created by students.