THE blue-green alga Anabaena catenula is a filamentous organism having specialized cells called heterocysts which appear at intervals along the filaments to form a spaced pattern1. The organism grows by division of all its unspecialized vegetative cells; heterocysts do not divide. The spacing between heterocysts is maintained by the appearance of presumptive heterocysts (proheterocysts) which develop from the vegetative cells roughly midway between previously formed heterocysts (Fig. 1). We have studied the growth of individual filaments of the organism (Cambridge Culture Collection strain 1403/1), grown on plates of a simple defined medium2 consisting of mono- and di-valent cations, phosphate and trace elements, solidified with 1% agar. From sequential observations of growing filaments, we have been able to obtain complete lineage records which show the fate of every cell over several generations. The fact that it is possible to obtain such information makes the organism well suited for studies of the process of cell division and of pattern formation. In particular we have observed that cell division is asymmetrical in A. catenula, and that this asymmetry is governed by a simple rule.