Teacher educators have focused on designing learning environments 'for' rather than 'with' teacher candidates. However, a co-design collaboration with teacher candidates has been recently suggested as a critical feature for effective teacher education programs. In this study, a co-design team, including preservice science teachers (PSTs) as co-participants of the iterative design process rather than research subjects, collaboratively developed formative assessment (FA) practices based on the approach of conjecture mapping in a chemistry laboratory setting. This study aimed to examine the role of PSTs as co-designers and their contributions to the progressive development of the most robust design of FA practices, drawing on relevant theoretical perspectives and considering the characteristics of a general chemistry laboratory environment. Sixteen PSTs participated in the study for a whole semester. This qualitative-interpretive study employed the following data sources: classroom observations, audio tape recordings of co-design meetings, and field notes. Results under four main themes showed that the PSTs as co-participants can actively engage in and effectively contribute to creating the FA design for their chemistry laboratory investigations. Using conjecture mapping as an approach for critical and reflective thinking on design elements, the PSTs could focus on their own thinking and that of others when making, testing, and revising co-design decisions based on the relevant theory work and empirical findings as the design principles for the FA-embedded laboratory practices. This study has important implications for the inclusion of teacher candidates as active co-designers of high-leverage instructional practices such as FA in science learning settings.