Since 1850, aphids have been regarded as harboring, in mycetomes, supposedly beneficial microorganisms which have, however, been the subject of some controversy. In the present study of the aphidsAphis jabaeandMacrosiphum pisi, mycetomes were identified in histological sections; but it proved impossible to isolate or culture any mycetomal microorganisms. Moreover, in a series of experiments in which great attention was paid to refined aseptic techniques no microorganisms could be isolated from the general body cavities or alimentary tracts of the aphids, though simultaneous attempts to isolate microorganisms from weevils were successful.