In order to examine possible variations in organic carbon isotopic ratios within a single Precambrian formation, the kerogen separated from 15 samples of the approximately 2000 m.y. old Gunflint Iron Formation and the conformably overlying Rove Formation, representing a wide range of lithologies and geographic localities, was isotopically analyzed. From the resulting data, four conclusions can be drawn: (1) δ13C values of the shallow water algal chert facies are significantly more negative (-25 to -30%.) than those of the deeper water chert-carbonate and taconite facies (-15 to - 20%). Comparative data for modern marine algal mats shows a range of δ13C values from -8.4 to -19%. PDB. Values obtained for fresh water mats were slightly more negative.(2) These differences in isotopic ratios can be correlated with similar differences in preserved microbiotas. (3) Anthraxolite lenses are depleted in 13C relative to the reduced carbon in surrounding sediments. (4) The effect of Keweenawan diabase intrusions upon the carbon isotopic composition is pronounced, but limited to the immediate vicinity of the contact.