A study examined the effect of three different prereading vocabulary development strategies on sixth grade science test results. Subjects, 81 sixth-grade students from a suburban, middle-class school in Union County, New Jersey, were divided into four heterogeneous science classes and given a pretest of 40 vocabulary words taken from the science textbook. Three of the four classes were guided in instructional prereading vocabulary development while the fourth group was given no prereading vocabulary treatment. One of the three experimental groups was taught vocabulary using the glossary strategy; another group used the context clue method; and the last group used the semantic mapping strategy. After approximately 4 weeks of study, the students were given the same vocabulary test. Results indicated no significant difference in test scores among the four groups. Results of the pretest indicated that the four groups were not equivalent in ability as previously thought based on random assignment to treatment samples. (Seven tables of data are included; the vocabulary test is attached.) (RS)