Stable isotope analysis provides an insight into feeding ecology and trophic interactions and is fast becoming a preferred, non-lethal tool in elasmobranch research. Slow turnover tissues, such as muscle, are representative of a long-term average (integrated) isotopic signal and do not represent recently ingested prey items. In comparison, tissues with fast turnover rates can be used to provide insight into seasonal dietary shifts that coincide with changes in foraging grounds. Validated fast turnover tissues in elasmobranchs include blood and liver, but these are difficult or impossible to sample in large free-swimming species. In bony fishes, mucus has been shown to be a rapid turnover tissue of value in dietary studies using stable isotope analysis, however, its utility in elasmobranch feeding studies is unknown. Here, we report the first attempt at processing mucus for use in stable isotope analysis for any elasmobranch species. Mucus and muscle tissue were collected from giant manta rays Manta birostris while on SCUBA. In comparison to muscle tissue the average mucus δ13C was depleted, and was similar to surface zooplankton δ13C values. No significant differences were found between muscle and mucus δ15N, indicating that M. birostris feeds at the same trophic level irrespective of a change in the source of dietary carbon. Results suggest that M. birostris mucus may be representative of a recent resource switch from a δ13C-enriched food web to surface zooplankton. The time period represented by isotopic values in different tissues is poorly understood for migratory elasmobranch species. Analysis of mucus may provide insight into aggregation site use by elusive and threatened elasmobranch species, however, controlled feeding studies are recommended to examine test the reliability of this approach.