Aquatic community structure is a reflection of the changes in constituent populations and the complex interactions between these organisms and environmental stressors. Consequently, shifts in populations and community structure can be used to assess water quality. However, these indicators only reflect damage already sustained by an ecosystem and are not useful for prediction of potential ecological impacts. Molecular/biochemical indicators, such as heat shock proteins, can provide early indication of environmental stress on aquatic communities. The heat shock protein response involves the synthesis of an array of proteins that protect organisms from cellular damage resulting from exposure to a variety of stressors. Consequently, stress proteins have the potential of being an important screening tool indicating exposure to, and/or biological effects of environmental contaminants. The midge larva Chironomus tentans has been extensively used in bioassays of freshwater systems. However, investigations of stress proteins as environmental biomarkers in midges are lacking. To evaluate the potential use of HSP-70 as a biomarker of environmental stress, we completed a preliminary characterization of the stress protein response in C. tentans upon exposure to heat shock. Western immunoblotting indicated an increase in a 72 kD protein after larvae were exposed to 33 °C, 35 °C, and 37 °C. The observed induction was rapid, appearing within 5–10 min, and persisted for over 24 h after removal of the stressor. The results are discussed with regard to the use of the HSP-70 biomarker as an environmental screening tool. It is proposed that the HSP-70 biomarker is most applicable as a sublethal toxicity test endpoint indicative of the presence of biochemically significant levels of stress.