Same- and mixed-gender small groups of middle school, seventh grade, students were observed gathering data on knowledge construction in terms of verbal communication and physical engagement in the biology laboratory. Video cameras with wireless receivers were used to record interactions between students in groups of four (36 target subjects in total). Both quantitative and qualitative methods were employed to examine how students exchange ideas, construct scientific concepts and perform laboratory work during laboratory sessions. Results indicated that gender differences exist in several verbal communication and physical engagement behaviour measures regardless of the same-gender or mixedgender groups. However, one or two students in either all-boy or all-girl groups were dominant in their small groups’ verbal communication and physical engagement, which suggested that individual differences also existed in the same-gender groups. It also showed girls working in the mixed-gender groups appeared to participate on a par with boys in the majority of engagement and verbal communication behaviours. However, more detailed analysis revealed that verbal communication and physical engagement patterns varied widely within the three mixed-gender groups. This study strongly suggested that girls have the potential to perform equally well as boys in the science laboratory and both individual and gender differences contribute to the students’ differential verbal communication and laboratory engagement.