1. The aim of this investigation has been to determine the sugar and nitrogen composition of the phloem sieve-tube sap ingested by Tuberolachnus salignus (Gmelin) feeding on Salix acutifolia stems, and to compare it with that of the honeydew excreted by the aphids.
2. A cage suitable for confining T. salignus on the willow stems is described.
3. Details are given of a technique, outlined by Kennedy & Mittler (1953) for collecting the fluid, termed stylet-sap, which exudes from the cut end of severed embedded stylet-bundles.
4. A method is described for collecting honeydew droplets immediately they are excreted by feeding T. salignus.
5. The nitrogenous matter ingested by T. salignus is in the form of free amino-acids and amides. The same amino-acids and amides are ingested but in greater amounts than they are excreted.
6. The number and concentration of the amino-acids and amides in stylet-sap and honeydew fluctuate with the seasonal development of the host plant.
7. The honeydew contains sucrose, fructose, glucose and melezitose. These sugars are derived from sucrose, the only sugar normally ingested.
8. The evidence for the identity of stylet-sap with the unchanged sieve-tube sap of the host plant is discussed.