This work is focused on the responses of some energy metabolism variables in Cnesterodon decemmaculatus adults exposed to cadmium under controlled laboratory conditions. This species has been used as bioindicator for evaluating the effects of different chemicals on diverse biological processes and is frequently used as test organism in ecotoxicity studies that include cadmium as reference toxicant. Animals were exposed for 12 days to the following concentrations: 0, 0.45, and 0.8 mg Cd/L. Food intake, fecal production, specific assimilation, condition factor, mortality percentage, oxygen consumption, oxygen extraction efficiency, specific metabolic rate, ammonia excretion, and ammonia quotient were measured. The overall balance was expressed as scope for growth (SFG). Cadmium-exposed groups showed a significant decrease in food assimilation and condition factor at the end of the exposure. There was an increase in specific metabolic rate and a decrease in SFG in the group exposed to 0.8 mg Cd/L. The condition factor and the SFG appeared as sensitive biomarkers of health status and growth of the animals, respectively. Cadmium-exposed fish reduced food intake, which was reflected in a decreased assimilation with concomitant decline in the external energy supply from feeding. Our results highlight the importance of considering the metabolic status of the test organisms when analyzing the responses of the biomarkers usually used as effect parameters in ecotoxicological evaluations under experimental conditions.