This study examined cell localisation and plasmid maintenance of Vibrio sp. S141(p519ngfp) cells which grew in, and detached from, monoculture and mixed-species marine biofilms under continuous flow conditions. Over the 48 h time course of the experiments, the broad host range IncQ RSF1010 derivative plasmid, p519 ngfp , was maintained in S141(p519ngfp) cells detaching from the biofilms irrespective of selection for the plasmid, the presence of another bacterial species, or the order of substratum colonisation. S141(p519ngfp) cell localisation within mixed-species biofilms was affected by the order, and length of time, of colonisation. When S141(p519ngfp) was the initial coloniser of the biofilm, the localisation of the majority of the plasmid-bearing cells near the substratum surface was not affected. When S141(p519ngfp) cells colonised a pre-existing Psychrobacter sp. SW5HR biofilm, or the two species simultaneously colonised the substratum, non plasmid-bearing cells initially dominated the substratum surface; as the time of S141(p519ngfp) colonisation increased, the plasmid-bearing cells appeared to displace the SW5HR cells from the surface. Since the p519ngfp plasmid was stably maintained in the Vibrio sp. S141 host forming biofilms over a 48 h period, GFP-producing S141 cells were able to be localised in mixed-species biofilms and were found to dominate the substratum surface by 48 h.