A one-year study (July 2003 – June 2004) to explore the spatial and temporal variability of potentially harmful dinoflagellates was carried out in the central coastal waters of Tanzania. During the course of the study a green dinoflagellate,Noctiluca scintillans, which has cosmopolitan distribution, was encountered for the first time in the coastal waters of Tanzania. It demonstrated a clear temporal distribution with high biomass (up to 30 cells/l) during the northern monsoon period and showed significant negative correlation with temperature and salinity. Noctiluca scintillans was possibly introduced by ballast waters or currents and became established for at least one season, but later disappeared due to the physical-chemical environment and/or competition or predation from other organisms in the water column. The need for a monitoring programme for potentially harmful micro-algae, and introduced alien species in Tanzanian coastal waters, is justified.