A study of the distribution of macrofauna and the ecological factors affecting their distribution in the bays and lagoons of the central and S Texas coast has made it possible to formulate a series of criteria for interpreting modern and ancient depositional environments. The observations reported in this paper cover a 7-year period. In addition, some information was available covering a period of 30 years. The central Texas bays are situated in a variable climate, and the faunas reflect long-term changes in rainfall and temperature. Four major environments are recognized on the basis of macro-invertebrate assemblages: 1) river-influenced low-salinity bays and estuaries characterized by Rangia and amnicolids; 2) enclosed bays, dominated by oyster reefs composed of Crassostrea virginica; 3) open bays and sounds characterized by Tagelus divisus, Chione cancellata, and Macoma constricta; and 4) bay and lagoon regions strongly influenced by inlets characterized by a mixed Gulf and bay fauna. Smaller "sub-facies" needing more information for recognition are 1) bay margins, 2) oyster reefs exhibiting marine influence, 3) bay centers, and 4) shallow grassy bays in the vicinity of inlets. Five assemblages were recognized in Laguna Madre which are related to the physiography of the Laguna Madre: 1) shallow hypersaline area near inlet characterized by forms common to the Gulf and normal salinity bays on the N; 2) open hypersaline lagoon characterized by Amygdalum papyria and other forms attaching to vegetation; 3) enclosed hypersaline lagoon with tremendous numbers of 2 pelecypods, Anomalocardia cuneimeris and Mulinia lateralis; 4) relatively deep hypersaline bay with clayey substrate with virtually no living macro-invertebrates; and 5) hypersaline lagoon with normal bay influence occupied by many of the species typical of an open bay plus Anomalocardia and Mulinia. The application of macrofaunal assemblages to the interpretation of older sediments was demonstrated in a study of a series of borings taken in the Rockport area. The macrofaunal evidence indicates that the Rockport bays have undergone at least one marine transgression in the past 9,000 years.