Flocs consisting of Anabaena and Zoogloea spp. were used as a model system for the study of planktonic phototroph-heterotroph interactions. In CO(2)-limited continuous culture (3.2 mumol of NaHCO(3) liter h, 1.5 mumol of glucose liter h, pH 8.5, D = 0.026 h), the biomass of the phototroph increased 8.6-fold due to association. However, direct CO(2) exchange accounted for only a 3.8-fold increase. When the glucose supply rate was increased to 7.5 mumol liter h, there was a 26-fold increase in biomass. When CO(2) was supplied in excess, there was no difference due to association. In batch culture, using the same medium, the specific growth rate was 0.029 h for the phototroph alone and 0.047 h for the phototroph in association with the heterotroph. The stimulatory effect of the heterotroph was found only under CO(2)-limiting conditions and was directly related to the concentration of organic matter supplied in the medium. Both the biomass and the growth rate of the Anabaena sp. were increased by association with the Zoogloea sp. Thus, dissolved organic matter may substitute for CO(2) to maximize both growth rate and biomass production by phototrophs when heterotrophic bacteria are present.