Chaetoceros muelleri Lemmermann was cultured in solution of different concentrations of Cu2+, Pb2+, Zn2+ and Cd2+, and in mixtures of any two of them. After 6 days the effect of these heavy metal cations on the growth rate and chlorophyll contents of the diatom were determined and an Opton microscope was used to observe the morphology and size of the cells.The experimental results show that the effects of toxicity of these four heavy metals on Chaetoceros muelleri are: Cu2+>Pb2+>Zn2+>Cd2+ . However, the cells did not differ significantly in their morphology and size from the controled ones.In any of the mixed metals cultures, its growth rate came close to and was higher than that of the single metal culture of higher concentration. Chaetoceros muelleri Lemmermann was cultured in solution of different concentrations of Cu2 +, Pb2 +, Zn2 + and Cd2 +, and in mixtures of any two of them. After 6 days the effect of these heavy metal cations on the growth rate and chlorophyll contents of the diatom were determined and an Opton microscope was used to observe the morphology and size of the cells. The experimental results show that the effects of toxicity of these four heavy metals on Chaetoceros muelleri are: Cu2 +> Pb2 +> Zn2 +> Cd2 +. However, the cells did not differ significantly in their morphology and size from the controled ones.In any of the mixed metals cultures, its growth rate came close to and was higher than that of the single metal culture of higher concentration.