Seventeen teachers and a university research team cooperated to compare the effects of team teaching and conventional instruction in biology on student achievement in six high schools in the Rochester, New York, area between 1964 and 1968. Student achievement was measured by the New York State Regents examination in biology, five locally developed unit tests administered during the course, and the Nelson Biology Test administered nine months after completion of the course. Data were treated by analysis of covariance using as covariables scores on the School and College Ability Test, Sequential Test of Educational Progress in Reading and Science, and a biology pretest developed for the project. Sex was also treated as an independent variable in some analyses. The results provided no evidence for the superiority of one method over the other, nor was there any evidence of a general improvement in biology instruction. Teachers' subjective evaluations of team teaching were studied by means of a questionnaire, and agreed advantages and disadvantages are listed. The report includes a review of the literature on team teaching, descriptions of scheduling arrangements, and the development of the unit tests and pretests. Appendices give the analysis of covariance tables and a copy of the teacher questionnaire. (EB)