This study investigated how a group of 10 prospective secondary mathematics teachers (PST) read, evaluated, and adapted a textbook lesson involving the symbolic manipulation capabilities of computer algebra systems (CASS). PST read the entire lesson and tended to focus on the organizing question at the beginning of the student lesson and the CAS-S sections of the lesson. PST frequently evaluated the lesson with respect to the teacher and whether lesson elements would promote student understanding. Only one PST evaluated the lesson with respect to the student and how the student might interpret lesson elements. There were five categories of curricular adaptations exhibited by PST: following, additive, mix, reductive, and adaptive. In general, the adaptations made to the lesson by PST were productive. These productive adaptations included asking students to make predictions before using CAS-S and understanding the hidden procedures used by the technology.