Flagellar development during cell division was studied in Cyanophora paradoxa using agarose-embedded cells, Nomarski optics and electronic flash photography. The cells bear two heterodynamic and differently oriented (anterior and posterior) flagella. Prior to cell division, cells produce two new anterior flagella while the parental anterior flagellum transforms into a posterior flagellum. The parental posterior flagellum remains a posterior flagellum throughout this and subsequent cell divisions. The development of a single flagellum thus extends through at least two cell cycles and flagellar heterogeneity is achieved by semiconservative distribution of the flagella during cell division. Based on these principles a universal numbering system for basal bodies and flagella of eukaryotic cells is proposed.