The surface cover (during 1998) and photosynthesis rates (measured intermittently between 1998 and 2000) of submergedcyanobacteria-dominated biofilms near a patchy reef in Zanzibar were determined using a line intercept transect method and pulse-amplitude modulated fluorometer (PAM), respectively. The biofilm surface cover ranged between 5 and 56% with an annual averagevalue of 25%. Photosynthetic activity on deep-dwelling biofilms was low light adapted compared to shallow-dwelling biofilms.Biofilms also regulated their photosynthetic activity depending on the light regime over the day, manifesting high light utilizationcoefficient (a), light saturation index (Ek) and maximum electron transport rate (ETRmax) values at around noon compared tomorning and evening measurements. We calculated carbon fixation rates of 0.05, 0.3 and 0.5 kg C m2y1for thin (w0.5 mm),medium (w1 mm) and thick (w2 mm) biofilms, respectively, and estimated an overall primary production rate of 0.14 kg C m2y1at depths of about 5 m. This study shows that biofilms in the area actively fix carbon and may contribute substantially to theprimary productivity of coastal ecosystems. More experiments are required to precisely determine the absorption factor for robustdetermination of the ETR and to explain the significance of the biofilms on the overall productivity of coastal ecosystems.