The objective of the present study is to evaluate a sensitive indicator of cadmium by measuring oxygen consumption and carotenoid concentration in post larvae of Penaeus monodon. The post larvae of P. monodon were exposed to 0.12 and 0.24 mg/L of cadmium in the laboratory for a period of 30 days. These concentrations represent 1/10th and 1/5th of 96 h LC50 value for the post larvae. Oxygen consumption and carotenoid concentration were measured at 24, 48, 96 h, 10 and 30 days of cadmium exposure. The results indicated an inhibition of routine oxygen consumption in post larvae at different intervals of exposure from 24 h to 30 days but the effect was statistically significant (P < 05) at 30 days of exposure at both sub lethal concentrations, where the inhibition was 38.5 % at 0.24 mg/L and 21.6 % at 0.12 mg/L. On the other hand, carotenoid concentrations increased gradually with increasing period of exposure as well as concentration, and this increase was statistically significant (P < 0.05) from 96 h onwards on exposure to 0.12 mg/L and from 48 h onwards on exposure to 0.24 mg/L of cadmium, with an overall highest increase of 135 %. As carotenoid content is an indicator of tissue oxygen, inhibition of routine oxygen consumption might have resulted in enhancing the levels of carotenoid. From the present data, it is evident that carotenoid concentration can be considered as a sensitive indicator of cadmium toxicity than routine oxygen consumption in the post larvae of P. monodon. The present investigation is also relevant for environmental monitoring of cadmium in coastal waters before it is used for aquaculture.