Aminolevulinic acid, the common precursor of linear and cyclic tetrapyrroles, can arise either by condensation of succinate and glycine or from the entire carbon skeleton of glutamate. Both pathways appear to operate in algae. In cultures of Anacystis nidulans, [l-14C]a-ketoglutarate labelled the linear tetrapyrrole pigment, phycocyanobilin, more efficiently than [1,4-'4C]succinate or [2-14C]glycine; [l-14C]acetate was even more efficiently utilized. When the culture was supplemented with [l-13C]acetate, 13C NMR analysis of the resulting labelled phycocyanobilin showed that only the carbonyl carbons of the pigment were labelled, indicating that the glutamate pathway operates in this organism.