This study examines the differential effectiveness of the laboratory method based on the investigative approach and the worksheet study, both used as a supplement to the regular class work on achievement in science subject-matter. Subjects were 43 students (8th grade) enrolled in two science classes of a secondary school. Each teaching strategy was randomly assigned to one class. The experimental group utilized supplementary instruction through the investigative-oriented activities; the control group received similar instruction through worksheet study. The Logical Thinking Ability Test was used as a pretest to control students' intellectual ability before treatment. The dependent variable was achievement in science subjects assessed through the use of the Science Subject Achievement Test. All subjects received instruction covering the same science topics during approximately five weeks. It was found that both treatment groups had statistically identical performance related to logical thinking ability at the beginning of the treatment. The subjects taught by the investigative-oriented laboratory activities earned significantly higher science achievement scores than those taught by the worksheet study. It was concluded that the supplementary instruction with the investigative-oriented laboratory activities seemed to be more suitable for teaching science subjects.