Identification and enumeration of phytoplankton species from Ilaje and Lagos sectors of the Nigerian coastal waters were conducted using standard procedures. Effects of different crude oil concentrations (6 to 50 ppm) on population of Coscnodiscus centralis, Thalassionema frauenfeldii, Odontella mobiliensis, and Ceratium trichoceros at different exposure periods (6 to 42 h) via microcosm experiments were then assessed. Results showed that the phytoplankton species consisted of diatoms (83.33%) and dinoflagellates (16.67%) whose abundance ranged from 2 to 516 Cell/mL. Crude oil toxicity varied from 0.06 to 36.43% for C.centralis, 1.41 to 35.58% for C.trichoceros, 1.71 to 46.11% for T.frauenfeldii and 0.66 to 44.90% for O.mobiliensis and showed direct relationship (r -+ 0.81 to +0.97; p < 0.001) with concentration but inverse with exposure period (r = -0.83 to â€“ 0.90; p < 0.001). Vulnerability within 24-h contact decreased in the order: T.frauenfeldii > O.mobiliensis> C.centralis > C.trichoceros. Study is a contribution to the scarce data bank on crude oil doseresponse assessment on plankton species in Nigeria, demonstrating that influx of crude oil into the Nigerian coastal waters is a risk factor to ecological status.