Cooperation between English as a second or other language (ESOL) and content-area teachers, often difficult to achieve, is hard to assess linguistically in a revealing way. This article employs register analysis (which is different from, but complementary to, genre analysis) in a Systemic Functional Linguistic perspective to show how an ESOL teacher uses the same content-area task as a cooperating science teacher so that she can provide a theory-practice cycle similar to that of the science teacher, but at a level that reflects and builds on the language abilities of her students. The task allows her to assess her students formatively and help them develop relevant meanings in the register of science. We argue that the development of register through related tasks in content classes and language classes provides a principled basis for cooperation and that register analysis offers revealing insights into cooperation and formative assessment between language and content teachers.