Science process skills are described as a set of broadly transferable abilities, appropriate to all of the science disciplines and reflective of the true behavior of scientists. While science process skills have gained wide acceptance as an integral part of the science curricula, the development of valid and reliable instruments to assess those skills has lagged behind. The purpose of this study was to gather evidence of the construct validity of the multiple-choice and performance-based versions of the Test of Basic Process Skills in Science (BAPS), the only research instrument designed to measure all of the most widely accepted basic science process skills for elementary and middle school students. A multitrait-multimethod construct validation technique was used to gather evidence of the convergent and discriminant validity of the BAPS tests. The BAPS multiple-choice test and the BAPS station test, a performance-based instrument, were used to measure the trait of interest. The Test of Logical Thinking (TOLT) and the Bending Rods (RODS) Piagetian manipulative task were used to measure the discriminant trait, science reasoning ability. The four instruments were administered to a sample of 151 seventh grade students from a west Florida school district. The results indicated strong support for the convergent and discriminant validity of the BAPS instrument. Considerable evidence of the construct validity of the BAPS tests can be inferred from this study. (Author/KR)