Biofilm formation is a major concern in any venture in the marine environment and often precedes the establishment of fouling by macro-organisms. In this study, the effects of three known cell-to-cell signalling molecules, nitric oxide (NO), cis-2-decenoic acid (CDA) and patulin, on the formation of marine biofilms were investigated. Each of the molecules has been shown to affect biofilms and this is the first study to investigate their effect on mixed communities of marine biofilm-forming micro-organisms. Studies of the biomass of those biofilms grown in the presence of the molecules showed that all three reduced biofilm formation by marine communities, with both NO and patulin reducing biofilm formation by more than 90% at the highest concentrations studied. However, colony counts revealed that the effect of patulin is likely due to toxicity. Analyses of the biofilm communities were also carried out using DGGE to determine whether there was any variation in the effects of each molecule on different bacterial taxa. No effect was seen on the communities and the biofilms appeared to form according to a neutral community model. Further investigations are recommended to study biofilms at a functional level.