This study had three purposes. First, to develop and validate a criterion-referenced test to measure science knowledge of students who finished the second cycle of the Basic General Education (6th grade). Second, to assess the performance of the entire Costa Rican population of sixth graders and, third, to analyze the results according to some selected variables. The tests were administered to the entire population of Costa Rican sixth graders. A total of 34,862 usable answer sheets were collected. Looking at the data by region, the same seven objectives were mastered by 50 percent or more of the students in 14 out of the 17 regions. Different results were obtained from comparing schools by size. Analyzing the means per objective, it can be concluded that the larger the school, the higher the means. (CW)