The carbon isotope records of the growth rings of three Wisconsin bur oaks (Quercus macrocarpa) have been studied. In addition a study of the earlywood and latewood of one tree showed no significant difference in the δ13C values of the separate components. A small, but not statistically significant correlation was found between the average δ13C values for the three trees and an average summer temperature. The δ13C values for the trees did not show a steady downward trend with time as has been suggested to occur because of proposed long-term increases in the concentration of atmospheric CO2 as the result of the burning of fossil fuels and forest clearing.