1. A study was made of the compounds in shrimp extracts which induce the proboscis search reaction in Nassarius obsoletus. 2. Compounds identified in shrimp extracts were as follows: alanine, asparagine, aspartic acid, betaine, glutamic acid, glycine, histidine, homarine, inosine, isoleucine, leucine, lactic acid, lysine, methionine, phenlyalanine, proline, serine, taurine, threonine, trimethylamine oxide, tryptophan, tyrosine, urea, and valine. Carnitine was tentatively identified. The amino acids and lactic acid were determined quantitatively; betaine was determined semi-quantitatively. 3. Glycine (ca. 10-3M) and lactate (ca. 5x 10-4M) were the only compounds identified in the extracts which possessed marked stimulatory capacities when tested individually; betaine (ca. 10-3M) was mildly stimulatory. However, quantitative analyses of these compounds in shrimp extracts showed that they were present in insufficient concentrations to account for the responses observed with the dilutions of extract which were employed. 4. Elutions of extract components from paper chromatograms revealed that eluates from large portions of chromatograms were more effective than eluates from small portions. This implied that the stimulatory capacity of the total extract stemmed from the combined effect of a number of compounds. 5. The response-inducing capacity of a combination of twelve compounds identified in the extracts (glycine, lactate, betaine, alanine, asparagine, aspartic acid, glutamic acid, glutamine, proline, serine, taurine, and threonine) was greater than was attributable to the response-inducing capacities of the individual compounds. This combination of compounds possessed a stimulatory capacity which approached, though it did not attain, the stimulatory capacities of shrimp extracts. The effectiveness of this combination of compounds was not increased by the combined addition of histidine, isoleucine, leucine, lysine, methionine, phenylalanine, trimethylamine oxide, tryptophan, tyrosine, urea, and valine. 6. N-Acetylglucosamine (ca. 5x 10-4M), 2-aminoethylphosphonic acid (ca. 5x 10-4M), glycogen (ca. 0.06 mg./ml.), and pyruvate (ca. 5x 10-4M) also possessed marked response-inducing capacities in N. obsoletus. Ascorbate (ca. 10-4M), citric acid (ca. 10-4M), glucose (ca. 5x 10-3M), and oxaloacetic acid (ca. 10-4M) were mildly stimulatory. 7. Compounds structurally related to N-acetylglucosamine, 2-aminoethylphosphonic acid, glycine, lactate, and pyruvate were either less effective or ineffective when tested at comparable concentrations. These findings suggest that considerable receptor specificity may exist.