The ends of arenavirus genome and antigenome RNAs are highly conserved and where determined directly, always contain a 3' G (referred to as position +1). However, primers extended to the 5' ends of Tacaribe virus genomes and antigenomes extend to position -1. When genomes and antigenomes are annealed either inter or intramolecularly and treated with RNase A or T1, there appears to be a single unpaired G at the 5' ends of the hybrids. A single extra G is also found by cloning the 5' ends of S antigenomes, and studies with capping enzyme detect (p)ppG at the 5' ends of genome and antigenome chains. A model is proposed in which genome replication initiates with pppGpC to create the nontemplated extra G. In contrast, the nontemplated bases at the 5' ends of the N mRNAs, which extend to positions -1 to -5, were found to be capped and also heterogeneous in sequence.