The mechanisms by which natural populations of vector mosquitoes cope with daily and seasonal fluctuations in temperature are poorly understood. We examined the effect of water temperature on expression of stress and immune-related genes in Aedes aegypti larvae. Aedes aegypti 3rd instars were exposed for 24 h to one of 7 constant temperatures (10°C, 15°C, 20°C, 25°C [control], 32°C, 36°C, or 40°C) and expression of antimicrobial peptides (cecropin, defensin), transferrin, and heat shock proteins (HSP70 and HSP83) quantified by real-time reverse-transcriptase polymerase chain reaction. Cecropin, defensin, and transferrin were overexpressed at 36°C and underexpressed at 15°C and 32°C. HSP83 was overexpressed at 10°C and 40°C and underexpressed at 20°C, while HSP70 was underexpressed at 15°C, 32°C, and 36°C. These findings suggest that antimicrobial peptides can serve as biomarkers of thermal stress and that HSP83 may buffer mosquito larvae against extreme temperatures.