SummaryDel13C-isotope ratios were determined in royal jelly samples harvested from 4Apis meillferacolonies at Pennsylvania State University. One sample, collected in July 1981, had a13C-isotope ratio of−13·3%0. This ratio indicates that the carbon of the dietary organic material utilized by the bees was predominantly from C4plants. The other samples, collected during August of the same year, had13C-isotope ratios ranging from—25·2 to−25·6%0; thus the carbon of the dietary sources of these bees was primarily from C3plants. The isotope ratios for 6 samples of commercial royal jelly products (pure royal jelly or royal jelly + honey) ranged from−6·9%0to−28·9%0. The possibility that some of these products were adulterated is discussed.