We examined simultaneous plastic and latitudinal interpopulation variation in the time course of hemolymph protein titers during egg production in the lubber grasshopper. Our goal was to gain insight into possible evolutionary changes in the physiology underlying reproductive plasticity. We used lubbers from three locations in the United States (Florida [FL], Louisiana [LA], and Georgia [GA]), each offered three daily food rations. Previous genetic analysis indicated that grasshoppers from FL (the low-latitude population) and GA (the high-latitude population) were phylogenetically closer to each other than to LA grasshoppers (the intermediate-latitude population). The ages at maximum titers of vitellogenin (Vg(max)) and three storage proteins that were referred to as major hemolymph proteins (MHP(max)) were used as indices of the progress of oocyte development. Age at Vg(max) was affected significantly both by diet and by population. Perhaps most importantly, age at Vg(max) was less for GA grasshoppers than for FL and LA grasshoppers; this pattern differs from the phylogenetic relationships of the populations. Age at MHP(max) was significantly affected only by diet and not by population. Hence, the regulation of these proteins may differ across populations. Finally, we found no evidence that plasticity of reproductive investment in response to food availability differs across populations (as indicated by nonsignificant interactions of population and feeding environment).