The development of selected science process skills in required and elective science classes in three high schools of a suburban Oregon school district was investigated. Science process skill knowledge in the areas of classifying, observing, interpreting data, identifying variables, modeling, and measuring was assessed by administration of an investigator-developed Science Process Competency Test (SPCT), a 24-item, multiple-choice test consisting of six subtests, one for each of the science process skill areas being assessed. In addition, other program and learner variables important in accounting for variance on the SPCT were identified. Subjects (N=900 grade 9-12 students) completed the SPCT, questionnaires, and California Achievements Tests (reading, language, mathematics) as a measure of academic achievement. The most important variable in accounting for SPCT variance was the level of academic achievement, with mathematics and reading achievement the strongest predictors. Other significant predictor variables included SPCT pretest score, sex, completing elective science, amount of elective science, elective physical science, type of required science, and school. Additional results are reported and implications/recommendations discussed. Supporting documentation (school district information, course statements, teacher materials/questionnaires, validation panel, SPCT, student materials/questionnaires, variable list, and a short essay on unified science education) are included in appendices. (Author/JN)