Abstract. A major problem for aphids is the avoidance of dehydration due to a high dietary osmotic pressure. Their adaptations include a high osmotic pressure in the haemolymph and polymerization of dietary sugars to oligosaccharides. The pea aphid, Acyrthosiphon pisum (Harris), was fed on an artificial diet containing Relabelled sucrose, and the fate of dietary sucrose was studied using quantitative paper chromatography. The haemolymph of A. pisum, feeding on artificial diet containing 25% w/v (730 mM) sucrose, contained two main sugars: trehalose (255 nw) and fructose (129 mM). No sucrose was found in the haemolymph. The honeydew sugars (350 mM) of aphids fed the same diet were mainly oligosaccharides (220 mM). The polymerization of sucrose was responsible for a 34% reduction in molarity of sugars in the honeydew. At low dietary sucrose concentrations, the honeydew contained mainly mono- and disaccharides. At dietary sucrose concentrations of 15% or more, oligosaccharides were predominant. This is consistent with the idea that osmoregulation is carried out by oligosaccharide synthesis. Analysis of the stomach contents revealed that oligosaccharide synthesis occurs there, and tissue incubation showed that die gut is much more active in oligosaccharide synthesis than the eviscerated body tissues. The function of the filter chamber, found in some aphid species, is considered and it is suggested that this is a mechanism for reducing the osmotic pressure of the ingested diet.