Evolution of photosymbiotic ecology is an important adaptation for planktic foraminifers that enhances the ecological advantage of living in oligotrophic oceans. Therefore, detecting photosymbiotic ecology in fossil species is one of the keys to understanding the paleobiodiversity dynamics of planktic foraminifers. Because foraminiferal tests record the ontogenetic history of ecological information in geochemical signatures, analyzing individual geochemical profiles with growth can reveal a species' ecology. This study examined chamber-by-chamber stable isotopes (δ¹³C and δ¹⁸O) of foraminiferal individuals to identify photosymbiotic signals. We observed an ontogenetic δ¹³C increase of up to 2.4‰, accompanied by relatively stable, negative δ¹⁸O, in the symbiotic species Globigerinoides conglobatus and Globigerinoides sacculifer. In contrast, δ¹³C and δ¹⁸O showed significant positive correlation during ontogeny in the asymbiotic species Globorotalia truncatulinoides. These two ecological groups produce contrasting isotopic profiles, thereby allowing us to use our ontogenetic isotopic analyses of individual specimens to identify algal photosymbiosis in fossil foraminifers. The chamber-by-chamber isotope analyses with individual ontogeny have great advantages in analyzing rare species because only one individual is required to describe ontogenetic isotopic history. In addition to photosymbiotic identification, our methods hold great potential to provide new insight into species paleoecological studies such as the ontogenetic history of calcification depth.