The toxic effect of metal ions like chromium (Cr3+), cobalt (Co2+), nickel (Ni2+), copper (Cu2+), cadmium (Cd2+) and lead (Pb2+) on biochemical oxygen demand (BOD) of synthetic wastewater samples has been studied at different temperatures i.e., 15 degrees C, 20 degrees C, 25 degrees C and 30 degrees C. Experiments were conducted for BOD exertion in presence (10 ppm of each metal ion) and in the absence of metal ions at different temperatures. Transition metal ions like Cr3+, Co2+, Ni2+ and Cu2+ show an increase in relative percentage inhibition with increasing atomic number. BOD inhibition in presence of Cd2+ and Pb2+ is relatively large. The metal ions under study are found to be highly toxic to microbes.