Aims: To investigate the effects of surface roughness and type of denture acrylic on the early development of a Streptococcus oralis biofilm in a constant depth film fermenter (CDFF). Methods and Results: Streptococcus oralis was incubated with acrylic of known surface roughness in the CDFF. Adherent Strep. oralis were enumerated by viable counting. Cold-cure acrylic was rougher (P < 0.01) than heat-cure acrylic after polishing with abrasive paper of any given grit-grade. Heat-cure acrylic was colonized by fewer (P < 0.001) bacteria than cold-cure acrylic at any given surface roughness. The number of bacteria adhering to heat-cure and cold-cure acrylic increased linearly with mean surface roughness after 2 h incubation, the increase being greater (P < 0.001) for the cold-cure compared with the heat-cure acrylic. However, after 4 h incubation, surface roughness appeared to have no effect on the number of adherent bacteria. Conclusions: The type of acrylic used, and its roughness, affect the early stages of biofilm formation by Strep. oralis. Significance and Impact of the Study: Choosing an appropriate type of smooth acrylic could lead to reduced biofilm formation in vivo.