This study examined the representational competence and fluency of preservice science teachers (PSTs) enrolled in a science teacher education program. It compared how these skills influence the understanding of the same cohort of PSTs when teaching concepts in chemistry and physics. Utilising a quantitative descriptive comparative design, the research analyses the participants' ability to effectively use multiple representations (MRs)--comprising graphical, experimental, symbolic, and verbal modes--during lesson presentations. Data from 39 PSTs were collected through video recordings that demonstrate concepts using various representation modes in chemistry and physics. Chi-square statistical analyses revealed significant differences in PSTs' graphical and experimental competence, with no significant differences observed in symbolic and verbal representations. The findings underscore the need for improved pedagogical strategies to enhance representational skills in science education, emphasising the interconnectedness of representational competence and fluency. It calls for targeted approaches in teacher education programs to better equip future educators with the necessary skills to foster effective learning outcomes in science classrooms.