TheAcharian genusCetraria has not been scrutinised in the same way as the genusParmelia in terms of circumscription of segregate genera. A few generic names have been introduced, but mainly in checklists without any real indication of what these names stand for. After a detailed investigation of characters in the apothecia, conidiomata, anatomical structures, as well as morphology and secondary chemistry, it is clear that several, more or less distinctive, phylogenetic lines exist among taxa which earlier were accommodated inCetraria. Some of these distinctive phylogenetic lines are distinguished by ascus characters: (1) clavate asci with a small axial body, an apical ring structure in the tholus and ellipsoid ascospores; (2) uniseriate asci with a large axial body and globose ascospores; and (3) taxa with broadly clavate asci with a large axial body and ellipsoid ascospores. Sixty-three character states were studied and results from the character analyses were evaluated by cladistic analysis against 43 examined terminal taxa. Seven further taxa were included in one analysis. Results from the cladistic analyses give rather strong support for a new systematic treatment of at least certain groups in the cetrarioid lichens. A formal systematic arrangement is not made here but will be presented elsewhere. The characters investigated are illustrated in 78 half tone pictures and one line drawing. Five cladograms are presented.