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The effect of insolation on body temperature of nymphal Packard grasshoppers, Melanoplus packardii Scudder, was measured in the field. Live nymphs were each restrained in a series of orientations to the sun, and insolation was adjusted using a shade cloth. Internal thoracic temperature was allowed to stabilize and was compared with that of a reference nymph restrained in a sunshade. Equilibrium body temperatures of insolated nymphs exceeded that of the reference nymph by an amount (Delta T-b) which increased with energy intercepted (ENERGY) and insect size (SIZE) by a relationship of the form Delta T-b = a . ENERGY . SIZE(b). When insect size was expressed as mass (grams), the estimates of a and b were 18.76 and -0.312, respectively (r(2) = 0.6198); when insect size was expressed as length (millimeters), a and b were 826.66 and -1.133, respectively (r(2) = 0.6463) These results allow estimation of equilibrium body temperature elevation of M. packardii nymphs from solar radiation, the zenith angle of the sun, insect size, and the orientation of the insect to the sun.