1. The ability of the green turtle (Chelonia mydas) to detect various chemical substances dissolved in water has been investigated using operant conditioning techniques. The turtles pressed underwater keys to obtain food reinforcement in the presence of a chemical stimulus.2. The turtles were capable of underwater chemoreception of β-phenethylalcohol, iso-pentyl acetate, triethylamine and cinnamaldehyde at approximate concentrations of 5 x 10-6 M or 5 x 10-5 M, but not of L-serine or glycine at an approximate concentration of 10-4 M.3. Stimulus generalization occurred when turtles were shifted from one test chemical to another.4. Intranasal injection of 0.35 M zinc sulfate solution interrupted olfaction for periods of from 1 to 5 days. Treatment with 0.35 M saline or magnesium sulfate had no effect on the performance of the chemical discrimination. It was concluded on the basis of these experiments that chemoreception in Chelonia is largely or entirely mediated by olfaction rather than by taste.5. The adv...