1 The construction of a raised dike across the coastal saltmarshes of Odiel (Huelva, south-west Spain) in 1977 divided a uniform area of low-lying sediment into two lagoons with very different drainage properties. To the east of the dike, there is rapid drainage into the estuarine channel of the Odiel and Tinto rivers, whereas to the west, the development of sand spits has impeded drainage such that standing water persists long after high tides, despite the slightly higher elevation of this area. 2 Both lagoons have been colonized by isolated clones of Spartina maritima, which have locally enhanced accretion to form domed tussocks (`nucleation'). The diameter of the tussocks was highly correlated with the elevation of their sediment surface, the larger tussocks having been the earlier colonists in a generally accreting system. Spartina in the interior of the tussocks showed reduced tiller density and vigour. 3 Only in the better-drained lagoon have the central, higher areas of the Spartina tussocks been invaded by Arthrocnemum perenne, although seed is freely available in both lagoons. Arthrocnemum formed a sprawling, dense canopy and a superficial, relatively impenetrable root system; it rapidly suppressed the remaining tillers of Spartina, eventually leaving only a fringe around the edge of the tussock. Areas invaded by Arthrocnemum were characterized by a superficial layer (10 cm) of oxidizing sediment $(E_h > + 80 mV)$. Spartina-dominated areas in both lagoons remained highly reducing, even in the surface layers ($E_h < - 150 mV$). 4 In terms of successional mechanisms, primary colonization by Spartina maritima facilitates the invasion by Arthrocnemum, which only becomes established from seed on relatively well-drained, oxidising sediments. The interaction is, however, more complicated than this: declining tiller density within the tussocks prior to invasion by Arthrocnemum is consistent with an inhibition mechanism, and the superior competitive ability of Arthrocnemum, once established, suggests that a tolerance mechanism may also operate.