Parameters of community structure (species composition and relative abundance, number of taxa, diversity, evenness, and cell density) were measured for a mudflat diatom community in the Avon‐Heathcote estuary, New Zealand. Fifty three diatom species were identified: 25 taxa (species and varieties) are new New Zealand records. The Shannon‐Wiener information index(H')was 3.46, indicating high diversity. Evenness (J') ranged from 0.57–0.67. The association between the biomass of the pulmonate gastropod,Amphibola crenata,and benthic diatom community structure was studied using large open enclosures (4.0 m2) to manipulate snail biomass. Community structure was compared at 0, 5 (natural biomass), and 10 gA. crenatadry weight per m. A similarity index (SIMI=0.88–0.95), as well asH’andJ’indicated close similarity between the diatom assemblages within all enclosures, but number of taxa increased from 33 to 49 with increasing snail biomass. Cell densities were significantly lower at high snail biomass (6088 valves per mm2) compared to enclosures with no snails (10 110 valves per mm ).A. crenatahad a higher ratio of diatom fragments to whole diatom valves in its faeces (2.42) than in its crop (0.55), indicating that it is capable of fragmenting diatoms.