In order to assess the classroom behavior of PLAN students relative to students in other instructional systems, the PLAN Student Observation Scale (PLAN-SOS) was developed. Specifically, PLAN-SOS was designed to measure the percentages of time that students spend performing various behaviors judged to be important to the learning process in a system of individualized instruction. Twenty categories of behavior were described, clustered under the general headings of either working alone, interacting in a group, interacting with the teacher, interacting with a peer, planning learning strategies, transitional behavior, or non-productive behavior. Two groups of observers were trained to use PLAN-SOS. The results these observers obtained using the scale are described in this report. A reference list and data tables are appended. (JY)