The sequences of the L1 loop of the hexon protein from representative fowl adenovirus (FAdV) strains of the different European and American collections were determined and compared. This study highlighted the lack of consensus in the numbering of the individual serotypes between the American and the European classifications. An identification system is proposed based on restriction fragment length polymorphism of the hexonA/hexonB polymerase chain reaction product. In addition, new insights into the relationships among FAdV strains are presented and discussed on the basis of phylogenetic analysis of the L1 loops sequences. Six clusters of strains that are supported by high bootstrap values were identified. Three of them are clearly independent, forming groups A, B and C, whereas the three others are clustered in a single 'supergroup', denominated D. Interestingly, the Japanese strain TR22 that is presently classified as European type 5 (species B) could not be assigned to any of the aforementioned clusters and might therefore constitute the sole representative of a seventh cluster.