Teaching methodology, such as problem-based instructional strategy, has arisen in response to educational research that has found evidence that even though lecturing may be the most prevalent teaching tool, it is less effective way to facilitate students learning. Problem-based instructional strategy is a teaching tool that teaches cooperative learning, grouping of students and an inquiry-based methodology for science education leading to better student learning, problem solving and retention. The researcher therefore conducted this 12-week experimental study to find out the relative effectiveness of problem-based and lecture-based instructional strategies in teaching the subject of General Science at secondary school level in (a) academic achievement, (b) retention of subject matter and (c) problem solving capabilities. It was hypothesized that there is no significant difference existed in increased average academic achievement, improved problem solving capabilities and increased average retention of subject matter between the students taught through problem-based instructional strategy and lecture-based instructional strategy. The researcher used purposive sampling technique for the selection of participants of the study. The participants of this research study were the 9 th class male students in Islamabad district taken from Federal Government Boys Higher Secondary School, Rawat, Islamabad and Federal Government Boys High School, Sangjani, Islamabad. The sample from the former school consisted of 67 students (N=33, N=34) and from the latter school, 41 students (N=21, N=20). For this study, pre-test post-test nonequivalent control group design was followed. Consequently, pretest, posttest and retention test were self developed and used as research instruments for measuring the variables of academic achievement, retention of subject matter and problem solving capabilities. Thus, the strategies were8 the independent variables and academic achievement, problem solving capabilities and retention were the dependent variables. The effect of problem-based instructional strategy was found out through its manipulation and was compared with the effect of lecture-based instructional strategy. The data were collected through administering the pre-test and post-test and were summarized by such descriptive statistics as mean and SD and analyzed by using such inferential statistics as t-test and Levene’s test. The level of significance used to test study hypotheses was .05. The results of the experiments conducted simultaneously in two secondary schools suggested that problem-based instructional strategy is not only more effective than lecture-based instructional strategy in facilitating increased average academic achievement of 9 th grade male students in the subject of general science but also in improved average problem solving capabilities and increased average retention. It is recommended that faculty development programmes regarding problem- based instructional strategy may be started and implemented so that General Science teachers may be prepared and supported to engage in the use of problem-based instructional strategy. The science teachers may apply problem-based instructional strategy in the classroom instead of lecture-based instructional strategy and consequently the learning of the students may enhance. Problem-based instructional strategy may be applied at secondary school level in science subjects as it enhances the retention rate of the subject matter and problem solving capabilities of students which is more desirable in their practical life. Similar studies be launched in other secondary school subjects and at other levels of education so as to generalize the results of this study.