The selection criteria of high intellectual ability and strong academic achievement for gifted educational programs often leads teachers to assume that their gifted students understand important science concepts. However, empirical evidence of such homogeneity among gifted students' deep understanding of scientific concepts is lacking in the research literature. The objectives of this study were: to describe the level of scientific reasoning ability of high school students in a gifted education program and to examine the viability of using analogous problems and questions designed to measure understanding of basic scientific concepts and skills. The results indicate that the development of valid assessments of science understanding is a key need for both individual student assessment and curricular evaluation. Another finding of the study, that even gifted students are not necessarily equal with respect to their ability to solve different kinds of scientific problems, supports the need for rigorous diagnostic assessment of students' conceptual science understanding and calls for increased small group or independent learning activities in science teaching. Recommendations from this study would be to assess gifted students' preconceptions of scientific concepts and to use multiple measures in judging students' scientific understanding since task-specific effects are very likely. (Contains 30 references.) (JRH)