The invertebrate infauna of Spurn Bight, an extensive area of mudflats in the outer Humber Estuary, internationally important for wildfowl and subject to land reclamation proposals, was studied in the form of a short term extensive survey of the wole area followed by a year long intensive study of the dynamics of the major species. Two-way cluster analysis of species and sample stations revealed three subcommunities within a, 'classic' boreal shallow mud 'Macoma' community distributed in relation mainly to sediment type and position on the shore. Ordination showed these to be continuous rather than discrete but the clusters were used as convenient units for extrapolation of single species' dynamics determined from the longer term study. Five major species: - Nephtys hombergi, Hediste diversicolor, Cerastoderma edule, Macoma balthica and Retusa obtusa were studied in detail and information on all other species was collated as an annotated list. For the major species, data on density, biomass, growth rate, mortality, production, elimination and productivity were analysed from the period March 1978 - March 1979. Mortality of certain species was found to be related to the very harsh winter and evidence was found of translocation of sediments and infauna within the Bight. Certain species, notably M. balthica, N. hombergi, R. obtusa and Eteone longa were found to 'perform' exceptionally well in the Bight and the total community biomass was found to be exceptionally high for a temperate silt ecosystem. The total invertebrate resource of the whole mudflat was estimated in conjunction with studies carried out on the avifauna over the same period and it is tentatively suggested that the mudflats may be able to support larger numbers of waders than at present. This is discussed in relation to the proposal for land reclamation and it is concluded that even partial schemes, designed to conserve part of the resource, would lead to the destruction of the most productive areas of mudflat in the middle and upper shore.