Starfish are significant determinants of community structure in marine ecosystems. The starfish Parvulastra (Patiriella) exigua is normally found on intertidal rocky shores. We examined a major outbreak of this species in Langebaan Lagoon, a marine embayment on the west coast of South Africa, and the only known soft-sediment ecosystem where P. exigua flourishes. We document this outbreak and describe subsequent field caging experiments testing the influence of this starfish on soft-sediment community structure. Grazing by P. exigua resulted in a reduction in microalgal biomass that was linearly related to starfish density, but it promoted bacterial density. Macrofaunal communities at intermediate densities of P. exigua were different and more diverse to those at high and low starfish densities, with Shannon-Wiener and Fischer's α indices peaking at intermediate density of P. exigua. It is possible that P. exigua at intermediate densities promotes co-existence of macrofauna by reducing the competitive edge held by dominant species. However, this effect may also be due to the balance between (1) negative effects of microalgal depletion and incidental grazing on invertebrate recruits at high densities of P. exigua, and (2) enhancement of bacteria by P. exigua leading to increased biofilm coverage on the sediment, which promotes macrofaunal recruit diversity in soft sediments. We suggest a need for caution when citing results from marine soft sediments as evidence for the intermediate disturbance hypothesis without knowledge of the promotive and inhibitory effects of the target organism. © Inter-Research 2010 · www.int-res.com.