The characteristic odor constituenta from boiled short-necked clam, clam, and corbicula were investigated. Some distinctions in the composition of sulfur-and/or nitrogen-containing substances were observed among the shellfish species. Pyrrolidino[ 1,2-e]-4H-2,4-dimethyl-1,3,5-dithiazine (I), which is a common component from cooked small shrimp, was also found in clam and corbicula. The presence of a large quantity of compound I should contribute a roasted odor to the taste of boiled corbicula. Compound I possesses an extremely low odor threshold. 1-Pyrroline and compound I were produced at the same time during heating and were increased by longer heating. It is postulated that compound I was formed secondarily from 1-pyrroline with ethanal and hydrogen sulfide in the shellfish during heating. INTRODUC~IONVarious kinds of shellfish are used as food materials throughout the world. Such shellfish as oysters (Ronald and Thomson, 1964;Josephson et al., 1985) and ascidian (Fujimoto et al., 1982) are preferred for their characteristic fresh aroma, but many shellfish are eaten after being cooked, because they give a pleasant and appetizing aroma when heated. In Japanese dishes, corbicula, short-necked clam, and clam are often used as ingredients of soup. Among them, corbicula is usually cooked with miso, which is a fermented product from soy beans, to mask the earthy odor of corbicula. As these bivalves each present a characteristic aroma of their own when heated, details of their cooked volatiles have been published by Nishibori et al. (1972), Kawai e t al. (1990), Gadbois et al. (1967), and Mendelsohn and Brooke (1968). However, no direct comparison among them can be drawn because the heating conditions were quite different. In this paper, the volatiles of boiled corbicula, short-necked clam, and clam were separated under almost the same conditions, and the compositions of the volatiles among them were compared.The formation of pyrrolidino[ 1,2-e]-4H-2,4-dimethyll1,3,5-dithiazine, which has been previously found in the volatiles of cooked small shrimps (Kubota et al., 1988), was also investigated to learn fundamental data about its formation mechanism in seafood. EXPERIMENTAL PROCEDURESSample Preparation. Three kinds of live bivalves, corbicula (Corbicula japonica, Lake Shinji, Japan), short-necked clam (Tapes phillppinarum, Lake Hamana, Japan), and clam (Meretrix lusoria, Mie coast, Japan), were used as samples. The bodies of the short-necked clams and clams were isolated along with the juice by hand-shucking. Corbicula was used without sucking the body from its shell. One kilogram of the edible part of samples with 1 L of deionized water was refluxed for 2 h in a modified Likens-Nickerson apparatus (Likens and Nickerson, 1964), and 50 mL of purified diethyl ether was used for extraction. After being dried with anhydrous sodium sulfate, the solvent was distilled at 39 O C to obtain the volatiles. The effect of heating time on the volatile components of corbicula was also investigated under the same conditions. Methyl ...