Green algae, Chlorella ellipsoidea when tested against 1st-stage larvae of Culex quinquefasciatus caused mortality of larvae up to 73% after 3 days. Very low mortality in the pupal stages was observed. Both field collected and laboratory cultured algae induced mortality of the same magnitude. Algal suspension caused the highest mortality followed by supernatant; little or none was caused by residues. The toxin(s) produced by the algae was lethal and also delayed the development of mosquito larvae. Mortality increased with the increase in cell density of both algal suspension and the supernatant to some degree. The highest cell density tested against 4th-stage larvae and pupae induced little if any mortality in the former and no significant mortality in the latter. The alga was maintained successfully in the laboratory without addition of CO2 and EDTA compounds.