This paper presents a literature review related to the time spent teaching science and a study designed to test the hypothesis that student achievement in science is related to classroom hours devoted to science instruction. The study involved testing two groups of eighth grade earth science students (N=200) over instructional material presented to one group where the subject is taught for a single semester and to another group where the subject is taught for two consecutive semesters. Both groups were given an identical 75-item multiple choice test covering such topics as plate tectonics, earthquakes, and volcanism. Statistical results (t-test) of this pilot study demonstrate that the group taught for two semesters scored significantly higher than the group taught for a single semester. It is noted that, since the study has not been repeated on other groups in different circumstances, the results should be considered as preliminary. However, should the same test be given over a period of time to many other students, and the results remain significant, then it can be inferred that the amount of material learned is related to the time spent teaching science. (JN)