Candida albicans is a pleiomorphic fungus that forms mixed species biofilms with Streptococcus gordonii, an early colonizer of oral cavity surfaces. Activation of quorum sensing (intercellular signalling) promotes monospecies biofilm development by these microorganisms, but the role of quorum sensing (QS) in mixed species communities is not understood. The comCDE genes in S. gordonii encode a sensor-regulator system (ComDE) which is activated by the comC gene product (competence stimulating peptide, CSP) and modulates expression of QS-regulated genes. Dual species biofilms of S. gordonii ΔcomCDE or ΔcomC mutants with C. albicans showed increased biomass compared to biofilms of S. gordonii DL1 wild type with C. albicans. The ΔcomCDE mutant dual species biofilms in particular contained more extracellular DNA, and could be dispersed with DNase I or protease. Exogenously added CSP complemented the S. gordonii ΔcomC transformation deficiency, as well as the ΔcomC-C. albicans biofilm phenotype. Purified CSP did not affect C. albicans hyphal filament formation but inhibited monospecies biofilm formation by C. albicans. The results suggest that the S. gordonii comCDE QS-system modulates the production of eDNA and the incorporation of C. albicans into dual species biofilms.