Aims The inherent instabilities associated with the development of multispecies biofilm communities within the constant-depth film fermenter (CDFF) and other microcosm systems can yield unacceptable variability between experiments, which could limit their potential applications in oral microbiology. The extent of this variability needs to be determined and a protocol developed which minimizes it. Methods and Results Two custom-made CDFFs were supplied concurrently with the same inoculation culture, begat from an aliquot of a saliva pool and artificial saliva growth medium via a dual-channel pump. Transformed log10 data of the viable counts at fixed time points were analysed using the BlandAltman approach to test for the levels of agreement between two CDFFs running concurrently and those CDFFs run in series. The coefficients95% of agreement were lower (i.e. less variable) in the concurrent model than when run in series for total counts of bacteria (1.238 vs 2.124), Lactobacillus spp. (0.517 vs 1.431) and Mutans streptococci (2.817 vs 3.864). Other measures of variability showed a similar trend. Conclusions Operating CDFFs concurrently minimizes the degree of difference and variability between them. Significance and Impact of the Study Operating CDFFs concurrently will improve the sensitivity for experiments that seek to determine the effects of a variable, such as a nutritional supplement or antimicrobial agent, and a control.