The Delaware, a major Coastal Plain estuary about 90 miles long, was quantitatively sampled for net zooplankton at quarterly intervals over a two-year period (#2 bolting cloth on Clarke-Bumpus samplers for 1-hour tows). The principal species were counted from 20-foot depth intervals and for 13 channel stations distributed from the Atlantic Ocean to fresh water. Accompanying hydrographic data were taken on salinity, temperature, dissolved oxygen, and surface transparency. The estuary always showed a gradient salinity from fresh water to about 31‰, but strong seasonal pulses responded to changes in runoff of fresh water. Temperatures varied from 3°C to 28°C in the river, with greater stability near the sea. Turbid river water cleared appreciably near the ocean, and dissolved oxygen levels were high except where pollution loading was apparent in the river. Both salinity and temperatures showed vertical stratification, especially during periods of maximum flow in winter and spring. The total standing crop of zooplankton was always greatest in the estuary, with smaller catches in the river and in the ocean. Seasonal levels of abundance per cubic meter were recorded for about 35 principal species. Among coelenterates,Aglantha digitale appeared in winter,Bougainvillia entered from the ocean in summer,Nemopsis bachei was produced locally in spring and summer, andBlackfordia virginica was sporadic. Ctenophores, especiallyMnemiopsis leidyi, were abundant and are suspected to be heavy predators of plankton. The chaetognathsSagitta elegans andS. enflata fluctuated seasonally but reached substantial levels of abundance. Catches occasionally included tomopterid and other worms, tunicates, isopods, larval and adult decapods, and fish larvae, but many of these were accidental and non-quantitative captures. Small crustacea usually comprised the catch in limnetic waters;Cyclops viridis and several cladocerans were important, especially in spring. The amphipodGammarus fasciatus showed massive concentrations in the oligohaline zone (ca. 0.5–5.0‰) during spring and summer. The relatively rich mesohaline zone (ca. 5.0–18.0‰) was always dominated by endemic estuarine species. The most abundant wereEurytemora hirundoides andE. affinis, Acartia tonsa (the most numerous, persistent, and euryhaline copepod in this estuary),Pseudodiaptomus coronatus andNeomysis americana. Polyhaline waters (ca. 18.0–30.0‰) characteristically contained low volumes of oceanic intruders, includingCentropages typicus andC. hamatus, Labidocera aestiva, Temora longicornis, Penilia avirostris, Evadne nordmanni, Pseudocalanus minutus, and others. A small number of species dominated the catch. Five species made up 89% of the volume (Gammarus fasciatus 54%,Acartia tonsa 22%,Eurytemora hirundoides andE. affinis 7%,Neomysis americana 6%). Three species provided 84% of the number caught (Acartia tonsa 65%,Eurytemora hirundoides andE. affinis 19%). These are the resident species which make the estuarine zooplankton distinctly different from the ocean and the river, and much richer than either. They offer promising fields for additional research. The volume of plankton was highest in spring and summer, and it was markedly affected by changes in the important species.Gammarus was responsible for most of the high spring and summer crop, andCyclops andEurytemora also varied greatly with season. Some stability was provided by the year-round presence of large numbers ofAcartia, Pseudodiaptomus, Centropages, Neomysis, and less abundant forms. The mechanisms which maintain holoplankters successfully in a flowing hydrographic system appear to involve specific patterns of behavior by individuals or by populations. Especially, these may provide adaptation to the two-layer system of circulation present in this estuary and in many others. Diurnal vertical migration, upstream sites of reproduction, species reservoirs in shoal lateral waters and other unknown mechanisms probably aid in successful estuarine existence.