A comparative analysis of glycolate excretion in 11 cyanobacteria showed that 8 strains, although grown and assayed in air, excreted glycolate. The largest quantities were excreted by the filamentous strains Plectonema boryanum 73110 and Anabaena cylindrica (Lemm). The carbon lost by excretion was at most 9% of the net fixed carbon in air for heterocystous cyanobacteria but increased (up to 60%) in some strains under a high pO2 (0.03 kPa CO2 in pure O2). A. cylindrica excreted glycolate at a maximum level of 2 and 10 μmol (mg chl a)−1 h−1 in air and at high pO2, respectively. The excretion continued for several hours. Increases in light intensity and pO2 and a shift in pH from 7 to 9 increased the amount of glycolate excreted. A. cylindrica also showed the most O2-sensitive fixation of CO2. In vitro activity of phosphoglycolate phosphatase (EC 18.104.22.168) was found in all strains tested, with the highest activities noted for Gloeobacter violaceus 7.82 and Gloeothece 6909 and for young cultures of A. cylindrica. The lowest activities were found in Anabaena 7120 and Anacystis nidulans 625, strains excreting no or only minor quantities of glycolate.