Two outstanding features appear in this study. One is the overall low measures of 13C and 18O in the banded iron- formation carbonates. The other is the statistically significant and locally pronounced alternation in isotopic composition between adjacent zones and microbands, with carbon and oxygen isotopically lighter where more iron oxide is present. A biological origin is favoured for the correlated variations in 13C and Fe. The low 13C of the carbonates may be derived from fermentative metabolism or from anaerobic respiration. -G.J.N.