Buchnera aphidicola is the prokaryotic, intracellular symbiont found in the aphid Schizaphis graminum. Using an immunological approach, we have quantitated the amount of the B. aphidicola chaperonin, GroEL, present in aphid cell-free extracts during the growth cycle of S. graminum at 23 degrees C. Our results indicate that the increase in GroEL approximately follows the increase in aphid weight and endosymbiont number for the first 12 days after birth of the aphid. A 9-day-old aphid contains 1.6 x 10(5) molecules of GroEL per mu m(3) of cell volume. This number is similar to that found in Escherichia coli growing at 46 degrees C, close to its maximal growth temperature, and a condition at which there is a major increase in the levels of chaperonins and other stress proteins. It is estimated that at 23 degrees C, 10% of the B. aphidicola protein is GroEL. When S. graminum grown at 23 degrees C was shifted to 33 degrees C for 1 day and subsequently to 23 degrees C, there was no change in the level of GroEL or the rate of growth. It is possible that the high level of GroEL in the endosymbiont masked an increase in the protein owing to a heat shock response.