<p>Stable isotope analysis of fishes generally employs muscle tissue, which is lethal for all but very large specimens. Non-lethal sampling of alternative tissues is desirable, especially for endangered species, within protected areas, or when repeated sampling of the same sites is required. Sampling of fin membranes is non-invasive and can substitute for muscle tissue in isotope analysis. Focus to date has been on freshwater species, with few marine fish validated. We compare δ¹³C and δ¹⁵N values from paired samples of muscle tissue and dorsal fin membrane in nine Mediterranean reef fish species. A mathematical lipid correction for high C:N ratios (&gt; 3.5, indicating high lipid content) increased fin δ¹³C compared to muscle in all species. In all but one case, muscle δ¹⁵N values were higher than those of fin. Fin-muscle offset values were generally consistent within species, indicating muscle isotope values can be estimated from fin membrane values using a constant correction or simple equations that account for variation with fish size. These results, together with an updated literature review comparing fin and muscle isotope values, support fin as a proxy for muscle isotope values but also highlights variation among species and continuing inconsistencies in the constituents of “fin” tissue analyzed.</p>