Penetration of the stems ofAraujia sericofera, Asclepias curassavica, Cynanchum ellipticum andSarcostemma viminale by stylets of the aphidAphis nerii was studied with light and differential interference contrast microscopes. Of a total of 118 stylets and 446 stylet tracks observed in cross-sections of stems of the fourAsclepiadaceae, 97 stylets and 372 stylet tracks terminated within the internal primary phloem. Of the remainder, 15 stylets and 74 tracks terminated within the external primary phloem. 22 stylets and 179 of the stylet tracks penetrated the external phloem on the way to the internal phloem. Of these, only four stylets and 32 of the stylet tracks showed signs of attempted probes of the external phloem. It is suggested thatAphis nerii may obtain not only its essential food requirements but also cardiac glycosides as a basis for chemical aposematism.