Small(≈cm)-scale spatial pattern of an abundant capitellid polychaete, Mediomastus ambiseta Hartman, was investigated in sediments of central Narragansett Bay, Rhode Island, USA from fall 1988 through spring 1992. Spatial autocorrelation techniques revealed a trend of decreasing patchiness and intensity of aggregation with increasing body size of M. ambiseta during the primary period of settlement and population growth (late summer-fall). The distribution and quality of particulate organic matter may explain the observed size-dependent spatial pattern. M. ambiseta influenced the small-scale distribution and concentration of organic matter and bacteria in surficial sediment. Experiments and field studies showed that this species creates two distinct sediment habitats through its sediment reworking activities: a surface pelletized zone enriched in organic matter and depleted in bacteria, and an underlying ‘feeding’ zone depleted in organic matter. Within the feeding zone of M. ambiseta in field-collected sediments, bacterial growth was enhanced. A density-dependent growth experiment suggested that small-scale aggregation in M. ambiseta may occur at the expense of individual growth.