Aggregations of Melarhaphe neritoides and Littorina saxatilis are common on the high shore in north-east England. These aggregations are frequently found in crevices which act as a refuge from desiccation. The aggregations are surrounded by patches of epilithic biofilm which are visibly lighter in colour than the biofilm present on the remainder of the high shore. Chlorophyll a levels were lower in regions of close proximity to aggregated littorinids (mean = 4.9μgcm−2) as compared to areas >‰5cm away from the edge of the visible colour change (mean = 15.9μgcm−2). Manipulations of littorinid density showed that areas of high density had significantly lower levels of chlorophyll a than those where littorinids were excluded. This difference was mainly due to the presence of higher numbers of small grazed areas or an increase in size of grazed areas rather than a homogenous change in chlorophyll a levels across the entire manipulated area. These results are supported by observations showing that only 7% of the total foraging time of littorinids was spent outside visibly lighter patches of biofilm suggesting littorinids only control local biofilm levels through grazing.