A study was conducted to identify the contributions of both prior knowledge and prereading vocabulary instruction to passage comprehension. In addition, semantic mapping and semantic feature analysis--instructional strategies that build upon students' prior knowledge--were compared with a modified basal approach for effectiveness as prereading instructional treatments for both vocabulary acquisition and passage comprehension. Fourth grade students from 13 classrooms were placed in either a full, partial, or control treatment group. Students in the full treatment group received prereading vocabulary instruction and read a basal passage prior to taking vocabulary and comprehension tests; subjects in the partial group either received vocabulary instruction or read the passage prior to testing; and subjects in the control group neither received vocabulary instruction nor read the passage before testing. Results showed that all three treatments were effective in teaching target vocabulary words. Significant gains were observed between the vocabulary pretest and posttest for the students receiving full and partial instruction. Findings also confirmed a strong relationship between prior knowledge and reading comprehension. Students with a high level of prior knowledge did well on the passage comprehension test regardless of treatment. While there were no significant treatment differences between subjects on the passage comprehension test, when they were grouped by prior knowledge level, there was a tendency for comprehension scores of students in the mapping and feature analysis groups to be higher than scores of those who received traditional instruction. (Lesson plan outlines are appended). (FL)