Annual variations in phytoplankton characteristics were studied from Bombay Harbour-Thana creek-Bassein creek (BHTCBC) estuarine confluence to assess the levels of pigment concentration, productivity and, qualitative and qunatitative nature of these microalgae during the premonsoon, monsoon and postmonsoon seasons. Of the 37 genera (33 diatoms and 4 dinoflagellates) recorded, Thalassiosira, Coscinodiscus, Nitzschia and Synaedra were most common. Maximum densityb of Skeletonema costatum (up to 93% of total cells) was observed during monsoon (June-September) months in the creek region which receive effluents from adjacent industrial estates. Chlorophyll alpha (av. 3.5 mg m sup(-3)) and phaeophytim (av. 4.2 mg m sup(-3)), cell counts (av. 631.9 x 10 sup(3) l sup(-1)) and productivity (71.4 mgC m sup(-3) d sup(-1)) were high and diversity index was low (H' 1.3) in the creek region compared to those recorded from the outer (coastal) stations, suggesting eutrophication in the former area. Effluents from the Bombay metropolis seem to have adversely affected phytoplankton diversity, abundance and productivity.