The purpose of this study was to determine the impact of learning various types of words in biology on students' reading comprehension, vocabulary performance, and science content knowledge. The study involved 315 ninth grade biology students who were placed in one of four groups and spent two weeks for ten minutes per day working on independent vocabulary packets in which they practiced a set of 15 words. Group one's list was a combination of domain-specific and general academic words, group two's list was a set of general academic words, and group three's list was a set of domain-specific words. The fourth group, the control group, did no formal vocabulary work but instead completed lessons involving the ecology content. In this quasi-experiment, the independent variable was the instructional group assignment, and the dependent variables were the students' performances on the reading comprehension, vocabulary (broken into various categories), and content assessments. Descriptive statistics for the majority of the vocabulary items and for the comprehension and content post-test measures revealed that the third group had the highest overall achievement. Throughout the two weeks of treatment, the third group worked only with domain-specific words related to ecology. A multivariate analysis of covariance (MANCOVA) found the differences to be statistically significant. The individual dependent variables were analyzed and found two question types on the vocabulary test, the domain-specific and general academic, to be significant in the test of between-subjects effects. Further, instructional group assignment did not have an effect on reading comprehension and content Descriptive statistics for the majority of the vocabulary items and for the comprehension and content post-test measures revealed that the third group had the highest overall achievement. Throughout the two weeks of treatment, the third group worked only with domain-specific words related to ecology. A multivariate analysis of covariance (MANCOVA) found the differences to be statistically significant. The individual dependent variables were analyzed and found two question types on the vocabulary test, the domain-specific and general academic, to be significant in the test of between-subjects effects. Further, instructional group assignment did not have an effect on reading comprehension and content. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]