Naturally produced methane shows different δ13C-values with respect to its origin,e.g., geological or biological. Methane-production of ruminants is considered to be the dominant source from the animal kingdom. Isotopic values of rumen methane—given in literature—range between −80‰ and -50‰ and are related to feed composition and also sampling techniques. Keeping cows, camels and sheep under identical feed conditions and sampling rumen gasesviaimplanted fistulae we compared δPDB13C-values of methane and CO2between the species. Referring to mean values obtained from 4 or 5 samples at different times of 11 animals (n= 47) we calculated δPDB13C-medians resulting in small but not significant differences within and significant differences between the species for CO2and methane. The δPDB13C-differences between methane and CO2were statistically equal within and also between the species. Therefore a linear regression of methane values on CO2is appropriate and leads to: δPDB13C(methane)‰ = 1,57 * δPDB13C(CO2)‰-47‰ with a correlation coefficient ofr= 0,87.