Studies of silicon (Si) cycling in estuarine systems have primarily focused on terrestrial inputs, benthic fluxes and uptake and recycling of Si by diatoms. However, intertidal marsh vegetation, which often covers a large area and contributes a high biomass to estuarine systems, also contains Si. Silica contents of two marsh plants, Spartina alterniflora and Juncus roemerianus, were measured in a mesohaline tidal marsh in the Cape Fear River estuary, North Carolina. Plant silica contents in the two species were significantly different in three age categories with new culms having the lowest and dead culms having the highest silica content. Mean silica contents in S. alterniflora were 4·1, 5·2 and 7·8mgg−1dry biomass in new, mature and dead culms, respectively. Mean silica contents in J. roemerianus were 0·67, 4·2 and 9·2mg−1dry biomass in new, mature and dead culms, respectively. Porewater dissolved silicic acid (DSi) was measured at three depths (0–10, 11–20 and 21–30cm) and was lowest in the upper 10cm. Mean porewater DSi concentrations were 198μM from January to May and 295μM from June to September. Sediment biogenic silica content (BSi) was 0·76 and 1·0% in J. roemerianus and S. alterniflora vegetative zones, respectively. Biomass estimates were used to obtain an estimate of the amorphous silica inventory of the marsh. Vascular plants and sediments contained approximately 7·1gSiO2m−2and 82·0gSiO2m−2, respectively. These data indicate that intertidal marshes may be an important reservoir of Si in estuarine systems.