Determined were (1) the effectiveness of a series of Biological Science Curriculum Study (BSCS) Single Topic Film Loops to improve, in five periods of instruction, the ability of high school biology students to construct hypotheses relevant to selected questions on the investigator's test, and (2) whether a relationship existed between students' success in relevant hypotheses construction and their I.Q.'s, their ability to interpret readings in the natural sciences, and their backgrounds in the natural sciences. Data were collected from 800 high school students enrolled in 30 biology classes in four high schools in south central Indiana during the spring of 1966. Concluded were (1) that relevant hypotheses construction was significantly improved through the prescribed teacher instruction which accompanied the use of five different BSCS Single Topic Film Loops, (2) that I.Q. was an important factor determining success in relevant hypotheses construction, and (3) that the students' background and ability to interpret readings in science as determined by score on the Iowa Test of Educational Development were important factors in determining success in relevant hypotheses construction. (RS)