A category system consisting of 36 categories was developed for classifying the verbal behavior of each pupil in high school biology lecture-discussion classes. Two classes each of eight high school biology teachers were observed and tape recorded for four consecutive days. Classroom interaction was coded combining a teacher-behavior category system with the pupil behavior system. The data were organized into interaction matrices, and frequencies of different modes of pupil participation were calculated. Behavior sequences were studied, and common cyclical patterns were identified. It was found that in the average classroom two pupils accounted for about 25% of pupil talk, four pupils for about 50% and eight pupils for about 75%. Pupil characteristics significantly related to participation were sex and previous achievement. Not related were intelligence (measured by Lorge Thorndike and California Test of Mental Maturity), School and College Aptitude Test scores, and attitude toward teacher, subject and class (measured by the Michigan Student Questionnaire). Final achievement in biology (measured by the New York Regents Examination and the teacher's final grade) was positively correlated with participation. (EB)