In scallop adductor muscle stored at 0 C, postmortem degradation of adenosine triphosphate (ATP), the predominant nucleotide at death, primarily follows the route ATP → adenosine diphosphate (ADP) → adenosine monophosphate (AMP) → adenosine → inosine → hypoxanthine (Hx). In one lot of scallops, however, inosine monophosphate (IMP) was present as an intermediate along with AMP. Hence, an alternate pathway, similar to that in fish muscle, whereby AMP is degraded to inosine and Hx via IMP rather than adenosine, may exist under certain (undefined) conditions. ATP dephosphorylation was much slower than in fish muscle; however, a rapid turnover of the intermediates in the degradative sequence, indicating relatively high activities of the enzymes catalyzing these reactions, resulted in the formation of appreciable amounts of Hx (3–4 μmole/g) within the edible storage period. In preliminary tests, taste scores correlated well with Hx contents, suggesting that measurement of Hx content may be a useful index of quality in scallop meats.Column chromatographic analyses showed that adenine nucleotides and their degradation products accounted for about 90% of the total acid-soluble purines and pyrimidines in scallop adductor muscle, with nicotinamide adenine dinucleotide and uridine diphosphate glucose as the principal minor nucleotide constituents. The muscle also contained considerable amounts of homarine (2–3 μmole/g). Glycolysis showed no relation to nucleotide degradation, both processes demonstrating a relatively slow postmortem catabolic rate in scallop muscle.