This study examined the relative importance of middle school students' explicit knowledge and tacit knowledge in carrying out open-ended laboratory investigations. The results indicate that the students provided with explicit knowledge through instruction performed significantly better in overall achievement in such investigations than those using only tacit knowledge. However, the performance over the duration of the study (7 weeks) of those students provided with explicit knowledge improved for some integrated process skills, while for other skills, the explicit knowledge gave no significant advantage over those provided with tacit knowledge. These findings have important implications on what teachers should emphasize when teaching open-ended laboratory investigations.