The study investigated the effects of different instructional methods and preinstructional backgrounds of the subjects upon the instructional effectiveness of Libyan high school teachers. The two measures of teaching effectiveness used in the study consisted of student learning as measured by the performance on a domain-referenced achievement test and the student ratings of course effectiveness. The sample for this study consisted of 15 tenth-grade classes in six public high schools located in the City of Tripoli, Libya. A total of 542 subjects participated in the study of whom 331 were male and the remaining 211 were female. The chronological ages of the subjects ranged from 13 1/4 to 28 1/6 years. Each of the selected participating teachers was randomly assigned two tenth-grade classes and was instructed to teach one class by lecture method and the other by discussion method. Thus, the same teachers each taught two different classes with varying instructional methodologies. Five classes taught by the other group of teachers called "non-participating teachers" served as the control group. The content of instruction was selected from a regular geography textbook and was divided into 12 units. Each unit represented a lesson. These lessons were delivered during 6 weeks of instruction at the rate of two lessons a week. Prior to the commencement of the study an achievement test, an ability measure, and an educational attitude scale were administered. At the termination of the study an alternate form of the achievement test and a course effectiveness scale were administered. The data collected were analyzed by using a factorial analysis of variance design. The ability, previous achievement, and educational attitude measures were used as indices of preinstructional backgrounds of the selected subjects. The obtained results indicated that the three instructional methods produced differential effects on subjects with varying aptitudes, especially when the overall previous achievement and ability test scores were used as indicators of learner aptitude. High-aptitude subjects showed the highest amount of learning gains and seemed to be the least affected by differences in the teaching methodologies. Subjects with medium aptitudes benefited most from the discussion method while subjects with low aptitudes profited most from the lecture method. These findings lend some support to the aptitude-treatment interaction phenomenon noted by other researchers. Similar aptitude-treatment interaction effects were not observed when educational attitudes, achievement in geography, and students' perceptions toward courses were used as measures of learner aptitude. The overall direction of the findings led to the conclusion that the effectiveness of different instructional procedures is related to individual characteristics of the learners, and that the aptitude-treatment interaction effects are observable only when global measures of ability and achievement are used as the indicators of the learner aptitudes.