Lygus hesperus Knight (Hemiptera: Miridae) is a key agricultural pest in the western United States, but certain aspects of its temperature-dependent development are poorly defined. Accurate models describing the relationships between temperature and development of L. hesperus would facilitate the study of Lygus seasonal population dynamics and overwintering ecology. We used nonlinear biophysical models to describe the development of L. hesperus eggs oviposited in green bean pods and semitransparent agarose and held under constant temperatures from 10 to 37.8 degrees C. The agarose substrate was used because it is less susceptible than green beans to deterioration at low and high temperature extremes, and because it allowed the observation of phases of embryonic development that are hidden from view from eggs developing in plant material. The models indicated that both low and high temperature inhibited development of each observed embryonic phase regardless of oviposition substrate. Developmental rates asymptotically approached zero with decreasing temperature in the lower thermal range, and decreased with increasing temperatures above 32.2 degrees C. Eggs oviposited in agarose developed slower than eggs oviposited in green bean pods suggesting that egg developmental rates were influenced by the type of host substrate. Our temperature-dependent developmental rate models for L. hesperus eggs supplement nonlinear models recently reported for L. hesperus nymphal and adult reproductive development and should be useful in planning and interpreting field studies of L. hesperus population dynamics and overwintering ecology.