Environmental exposures to toxic metals result in ecological effects that are difficult to detect and quantify. There is a pressing need for biomarkers that can be used in the field and that are sensitive to the sublethal metal levels that are prevalent in the environment. Using a combination of molecular and demographic techniques, we developed a biomarker to detect arsenic stress in freshwater zooplankton. We measured changes in HSP 83 mRNA levels by RT-PCR analysis in Daphnia pulex adults and juveniles exposed to arsenite and arsenate, and we compared the HSP response to demographic effects on Daphnia survival, reproduction and development. HSP 83 appears promising as a biomarker for arsenic stress given that arsenic exposure induced an increased and sustained expression of mRNA. Both molecular and demographic measures of arsenic stress revealed differences between adult and juvenile responses and toxicity of arsenate and arsenite. The molecular responses are more sensitive than the demographic response of individuals exposed as adults but corroborate the reproductive responses for individuals exposed as neonates.