A number of studies have recently shown that the hacmolymph proteins of Insects change during development. Most of the work in this field has however been done on holometabolous Insects, while only a few studies have been made on the Hemimetabola (STEINHAUER and STEPHEN, 1959 on Periplanefa ; MISSELIJN, KARCHER, DE KEYSER, and VAN SANDE, 1959 on Trialornu; DUKE, 1966 and MCCORMICK and SCOTT, 1966 on Locusta). There have been additional studies of haemolymph proteins of various imaginal hemi- and holometabolous Insects, but these are not entirely relevant here. The electropherograms prepared by the above workers represent haemolymph samples chosen usually at one unspecified time during the various instars. No investigation of the ontogenesis of haemolymph proteins has been found which follows the protein changes throughout the entire life history at regular short periods of time, e.g. at 1 day intervals. The cyclic nature of the Insect life history suggests that such a study of haemolymph protein fractions would be of considerable interest in view of our increasing knowledge of metabolic changes associated with development in the Insect. This approach has already proved useful in having shown the presence of a protein fraction whose cyclic appearance throughout the ontogeny of the locust suggests a strong association with a moulting hormone (MCCORMICK and SCOTT, 1966). The present investigation sets out to examine