The purpose of this study was to (1) develop and validate criterion-referenced tests to measure science knowledge of students who finished the fifth grade, as well as those who finished the three cycles of the General Education and (2) to assess the performance on these tests of a national, random sample of fourth-, sixth-, seventh-, and tenth-grade students belonging to 127 public elementary and 43 public secondary schools in Costa Rica. Results show the internal consistency of the tests (calculated using KR-21) to be 0.78, 0.68, 0.83, and 0.80 for the fourth-, sixth-, seventh-, and tenth-grade levels respectively. Additional results (obtained from administering tests to 1,012 fourth-grade, 1,130 sixth-grade, 794 seventh-grade, and 449 tenth-grade students) show very low achievement in science. If the results are interpretable in terms of the objectives which were mastered by samples of students, the results are very disappointing, because of the efforts made in the last 12 years by several government agencies to improve science teaching in Costa Rica. In addition, the even lower achievement of students in some regions of Costa Rica pointed to the lack of well-prepared teachers and science materials that were found in another study conducted in those same regions. (JN)