Studies on the feeding habits of postlarval abalone in their natural habitats have never been carried out primarily because of the lack of an investigating tool. In this study, carbon and nitrogen stable isotope ratios (δ¹³C and δ¹⁵N) were tested to detect nutritional sources during the early life stages of the abalone Haliotis discus hannai in laboratory experiments to evaluate stable isotope analyses as an appropriate investigative tool for field studies. Fertilized eggs, larvae, and the parent spawner muscle were analyzed to detect a nutritional source during the nonfeeding period. The δ¹³C of fertilized eggs showed slight depletion, about 2‰, compared with that of the parent muscle, and was not detectable after lipid extraction of fertilized eggs. As for the larvae, the isotopic values in postlarvae after settlement and metamorphosis were 2‰ more enriched, become similar to that of the spawner muscle. These results imply that the lipid portion of the eggs is the primary nutritional source during the nonfeeding period of abalone. Laboratory feeding experiments were conducted during which early postlarval abalone (approximately 1 mm in shell length (SL)) were reared for 7 days and supplied with 4 benthic diatoms respectively. The benthic diatom Cylindrotheca closterium produced high growth and survival rates for early postlarval abalone, and there was a significant isotopic shift in their δ¹³C, approaching that of C. closterium. Larger postlarval abalone (SL, >1.3 mm) were fed 1 of 5 benthic diatoms and a gametophyte of a macroalga, Undaria pinnatifida, for 21 days. Isotopic changes approaching the δ¹³C of the provided benthic diatoms were detected from postlarvae fed the benthic diatoms Achnanthes longipes, Cocconeis scutellum, C. closterium, and the gametophyte of U. pinnatifida, which provided high survival rates and growth rates for the postlarvae. From these results, stable isotope ratios of postlarval abalone are suggested to be useful to infer the origin of the assimilated diets. The current study highlights that stable isotope ratios are a promising tool to investigate the diet composition of postlarval abalone in their natural habitat as well as in abalone hatcheries.