ABSTRACT: The mussel Musculista senhousia is capable of marked habitat alteration through the construction of byssal mats on the surface of soft sediments. Here I demonstrate the importance of this alteration on sedimentary properties and residentmacrofaunal assemblages of a tidal flat in Mission Bay, San Diego, California, USA, where the mussel is exotic. In well-developed mats, percent fine sediments, percent combustible organic matter, and sediment shear strengths were increased relative toadjacent areas without mats. Comparisons of naturally occurring areas with and without mats of M. senhousia, as well as comparisons of the same tidal flat when mussels were seasonally present and absent, revealed that assemblages within mussel matsdiffer from those in sediments without mats. The primary effect of the mussel and its mats was facilitation of other organisms. Total densities of all macrofaunal individuals as well as species richness were typically higher inside than outside musselmats. Two species that exhibited large enhancements of densities within mussel mats were the tanaid Leptochelia dubia and the gastropod Barleeia subtenuis. Oligochaetes, a numerically important component of the tidal flat, appear leastfacilitated by the presence of mats. A short-term, manipulative experiment that examined the effects of mussel mat mimics on the colonization of macrofauna suggested that the presence of physical structure alone can produce several of the patternsobserved in naturally occurring mussel mats. These results highlight that alteration of habitats is an important effect of exotic species, and that these habitat alterations can have subsequent effects on resident biotic communities.