Investigations of growth rates of marine mollusks are important for understanding paleoenvironmental conditions that influenced their evolution. Here we obtained oxygen isotope (δ¹⁸O) profiles for two Umbonium (Suchium) giganteum individuals collected from Sagami Bay on November 25, 1999, to verify the findings of a previous study that used population dynamics to determine growth rates. The oxygen isotope profiles for both individuals exhibited similar cyclic patterns of gradual increases and rapid decreases, with three maximum and three minimum values. Because this species inhabits areas that are not influenced by fresh water, these values represent seawater temperatures from spring 1997 to fall 1999. Profiles of both individuals were smooth in both summer and winter, suggesting that this species grows almost continuously throughout the year, with no major seasonal cessation of growth. The findings suggested that the prominent growth lines on the surfaces of the shells coincided with sudden decreases in the seawater temperature. Because this species mainly spawns in fall and winter when the seawater temperature decreases, these prominent growth lines can thus be considered to represent spawning events. This was supported by observations of other individuals that were collected at the same time, which suggested that spawning occurs once or twice each fall. However, spring or early summer spawning was inferred from the oxygen isotope analysis of microsamples from the older parts of one of the individuals, which would likely not have been recorded as prominent growth lines because of the greater growth rate during this season. Our results broadly confirmed those of a previous study on the rate and pattern of shell growth of U. (S.) giganteum. However, we found slightly higher growth rates than those reported by the previous study, particularly in juveniles.