Loss of saltmarsh vegetation in south-east England is a significant problem for conservation and coastal defence. The losses of vegetation began in the 1930s with the loss of intertidal Zostera marina and have continued more recently. Some preliminary trials at reestablishing Zostera in some estuaries of Essex have not been successful. This paper addresses the hypothesis that the infauna, particularly the polychaete Nereis diversicolor, may restrict natural colonisation by Zostera and reduce the success of transplanting trials. In field experiments, Z. noltii were transplanted into areas where Nereis were common, close to an established seagrass bed and into two other estuaries. The transplants protected from the effects of the polychaetes by netting had a higher survival rate, lower index of root damage and greater biomass at the end of the experiments than those that were unprotected. In laboratory experiments, Nereis reduced the survival of Z. noltii. They were observed grasping the leaves and pulling them into their burrows. These results indicate that herbivory and disturbance by N. diversicolor is responsible, at least in part, for the restriction of the distribution of Z. noltii and may have been important in limiting the success of previous transplanting experiments. A hypothesis is proposed which states that there are two stable states on the upper mudflats. One state is dominated by plants, including Zostera spp., which prevent colonisation by burrowing infauna, and the other is dominated by infauna which prevent colonisation by plants. Managing these two states could be the key to re-establishing the early successional stages of saltmarsh development.