Monthly phytoplankton samples were taken in the lower Cape Fear River estuary at three stations from the estuary mouth to 8 km upriver. Surface salinity ranged from 16 to 35‰ near the estuary mouth and from 2 to 26 ‰ 8 km upriver, and surface water temperatures ranged from 4.5 C in January to 30.0 C in July. A total of 203 species were observed in the estuary, consisting of 134 diatoms, 25 chlorophytes, 15 dinoflagellates, 9 cryptomonads, 9 cyanophytes, 3 chrysophytes, 3 xanthophytes, 2 haptophytes, 2 loricate flagellates, and 1 euglenoid flagellate. The phytoplankton was dominated by the diatomSkeletonema costatum, which averaged 28.7% of the total cell numbers through the year.Skeletonema costatum, and five other species consisting of 2 diatoms, 1 cryptomonad, 1 dinoflagellate, and 1 loricate flagellate, were the major phytoplankters. Average cell numbers in the estuary were 1.69 × 106 cells/liter, and the maximum and minimum were 7.3 × 106 and 0.25 × 106 cells/liter. The major population bloom was in May and June in 1969 and appeared to start in February in 1970. Phytoplankton species diversity was calculated with the Shannon formula and showed greater average diversity at the mouth of the estuary.