On the basis of bulk carbonate carbon isotope data from six drill cores and the type section of the Porkuni Stage of Estonia, δ 1 3 C curves were compiled, which altogether cover nearly the whole succession of the latest Ordovician (Hirnantian) -well known as a time of short-lived glaciation and mass extinction. In pre-Porkuni and Silurian rocks, δ 1 3 C values were low (from -0.2%% to 1.7%%). For the Porkuni Stage three intervals of the curve were established: (1) lower - a positive shift up to 6.7%%, (2) middle part with variable values from 3.3%% to 5.4%%, and (3) upper phase with slightly decreased values from 3.2%% to 3.6%%. The correlation with the Dob's Linn (Scotland) kerogen δ 1 3 C curve shows that the first two intervals coincide with the Normalograptus extraordinarius graptolite Biozone, the third one with the N. persculptus Biozone.