Abstract—Studies were carried out on the toxic effects of cadmium on the sewage quality and microbial diversity of oxidation ponds. It was found that wastewater treatment efficiency was affected by the presence of cadmium. On day 16, the level of BOD reduction recorded was 95.7%, 92.1%, 79.7%, 38.3% and 34.8% and the level of phosphate reduction recorded was 68.2%, 65.9%, 54.5%, 42.0% and 31.8% in control, 0.5 mg/L, 1.0 mg/L, 5.0 mg/L and 10.0 mg/L of cadmium treated oxidation ponds respectively. The level of pH, DO, and catalase enzyme activity were decreased and the level of TDS, BOD and phosphate concentrations were increased as the concentration of cadmium was increased in the oxidation pond. Microbial density recorded was maximum in control and 0.5 mg/L of cadmium treated oxidation ponds compared to other ponds treated with higher concentrations of cadmium. The algal species belonged to the genus Anacystis and the species Chlorella vulgaris showed more tolerance, whereas the species Zygnema pectinatum , Scenedesmus acuminatus and Euglena viridis were found to be more sensitive to cadmium toxicity. Among protozoans, the species of the genus Spasmostoma and Onchomonas showed more tolerance and the species Podophrya fixa, Pelomyxa palustris and the species belonged to the genus Thylacomonas were showed more sensitivity to cadmium toxicity.