In this experiment, 376 undergraduates studied five instructional units through the audio-tutorial method during a five-week period. Instructional unit sequences included a Structure Based Unit Sequence (SBUS) and a Test Determined Unit Sequence (TDUS). The SBUS was based on an analysis of major concepts; units including concepts considered prerequisite to other units were put into the sequence on that basis. The TDUS began with the unit on which students scored highest on an achievement pretest and moved progressively to units represented by lower mean achievement scores. The two forms of control for within-unit experience sequences were Teacher Directed Experience (TDE) and Student Selected Experience (SSE). TDE subjects followed a teacher-prescribed sequence while SSE subjects used lists of objectives and related activities to select within-unit sequences. The findings included: treatment groups did not differ significantly in biology achievement or attitude toward the biology course; subjects in all treatment groups made highly significant gains in achievement; and SSE subjects used significantly more time than TDE subjects on two instructional units. A combination of factors which included SAT-mathematics score, grade point average, and the score from a biology achievement pretest were the best predictors of biology achievement. (Author/MH)