An oil spill was simulated in microcosms in the laboratory using phytoplankton assemblage collected from Versova, Mumbai. Bombay High Crude oil, water-soluble fraction and water-dispersed fraction were added to respective tanks. Their effect was investigated on various parameters like primary productivity, growth and biomass of the phytoplankton at different time intervals. There was reduction in primary productivity and cell number resulting in decreased biomass after 6 h, 24 h and 48 h.