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A classical conditioning technique was employed with cod, Gadus morhua L., to determine thresholds for the detection of the L-forms of some α amino acids which are thought to be attractants or feeding stimulants for fish. The amino acids investigated, in order of effectiveness, were tyrosine, cysteine, phenylalanine, glycine and methionine with mean threshold response levels ranging from 2·5 × 10-8M to 7·4 × 10-8M. Histidine and lysine resulted in similar thresholds with a mean value of approximately 3 ± 10-7M while taurine and leucine were least effective with mean threshold levels of 2·1 × 10-6M and 2·1 × 10-5M respectively. Comparison is made with electrophysiological and behavioural response data from other species. The effect of raising the background level of glycine on the threshold to glycine for cod is described. The results are discussed with reference to data on levels of dissolved free amino acids in shallow sea waters which may have a bearing on determining chemosensory threshold levels. To detect a specific amino acid against a background level of the same substance the difference in level for detection is proportionally greater for higher background concentrations.