ABSTRACT.The influence of nucleic acid constituents on the rate of ingestion of charcoal powder filtered from ambient water by larvae ofCulex pipiensL. was examined. All nucleotides tested stimulated ingestion to some extent. Various mono‐, di‐ and tri‐phosphates of adenosine were most effective and at concentrations of 1 mM stimulated ingestion nearly as well as yeast extract, a powerful phagostimu‐lant. Guanylic, thymidylic, cytidylic and uridylic acids were less stimulatory, the latter two even at 10 mM. Cyclic AMP and deoxyadenylic acid were less effective than other adenine nucleotides. The nucleosides adenosine, guanosine and uridine were almost as effective as their corresponding nucleotides (adenylic, guanylic and uridylic acids); thymidine was less effective than thymidylic acids, whereas cytidine was non‐stimulatory. Adenine, guanine, uracil and cytosine, the bases of the ribose nucleotides, were non‐phagostimulatory, whereas thymine, base of the deoxynucleotide, thymidylic acid, caused low but significantly increased ingestion. These findings are compared with the reported phagostimulation by nucleic acid constituents of certain plant feeding insects and with the stimulation of engorgement of the blood meal by many blood feed