This study measured Mg/Ca values in cross sections of the benthic foraminifer Planoglabratella opercularis (d'Orbigny) using an electron probe microanalyzer (EPMA) (spot size, 2 ¿m; >1000 analyses). We cultured clonal juveniles of this species under controlled temperatures. This species precipitates high-magnesium calcite as its test material. Mean Mg/Ca values increased linearly with the culture temperature; therefore Mg/Ca values in walls of subsequently formed chambers reflected changes in temperature during the experiment. Foraminiferal specimens yielded variable Mg/Ca values within their test walls, which resulted from the pattern of chamber formation and from minor variations of Mg/Ca within one chamber wall.