A series of 48 vertical plankton tows taken in January, May, and September of 1976 and 1977 in the south Texas outer continental shelf waters were used to study the distribution and ecology of shelled microzooplankton. Species composition and abundance, number of species, and total standing crops of foraminiferans, radiolarians, and pteropods were correlated with physical, chemical, and biological oceanography of the study area. Using shelled microzooplankton distributions, seasonality, nearshore and offshore waters, estuarine and river runoff, upwelling and current directions can be identified on the shelf. Seasonality in shelf waters is suggested by the presence of two distinct assemblages: a cold to temperate water assemblage in January and a temperate to warm*water assemblage in September. January samples are dominated by the planktonic foraminiferan species Globigerina falconensis, Globigerina quinqueloba, Globigerina inflata, Globorotalia menardii and Globorotalia truncatulinoides and the polycystine radiolarian species Euchitonia furcata arid Cladococcus scoparius. Species in abundance in September include the planktonic foraminiferan species Globigerlnoides ruber and Globigerina bulloides and the polycystine radiolarian species Botryosyrtis scutum, Euchitonia elegans, Pterocorys zancleus and Hymeniastrum profundum. Nearshore and offshore conditions in the study area are suggested by the densities and diversities of planktonic foraminiferans and polycystine radiolarians and the ratio of nassellarian to spumellarian radiolarians. Maximum densities and diversities are found at the outer shelf along with the highest ratios of nassellarian to spumellarian radiolarians. Nearshore waters are characterized by low densities and diversities of microzooplankton in addition to an abundance of spumellarian radiolarians. A high influx of estuarine and river runoff water onto the shelf is suggested by the abundance of the benthonic foraminiferan species Bolivina lowmani found live in plankton tows, the pteropod species Creseis acicula, and the acantharian radiolarians. High volumes of runoff water are also indicated by the low densities and diversities of planktonic foraminiferans and polycystine radiolarians. Upwelling and ponds or rings of offshore water on the shelf are suggested by the presence of species commonly found at depths greater than the shelf break associated with water masses in the Central Gulf including Globigerinoides sacculifer, Globorotalia scitula, Spongotrochus glacialis, Theoconus hertwigii, Amphirhopalum ypsilon, and challengeriids. The south Texas outer continental study provides an example of how shelled microzooplankton in particular the foraminiferans and radiolarians can be used successfully as hydrological indicators of oceanographic conditions.