Viral RNA from fall armyworm (Spodoptera frugiperda) cells infected with Autographa californica nuclear polyhedrosis virus contains cap structures. Most of the cap labeled in vivo with [3H]methionine or 32pI cochromatographed on DEAE-cellulose with the -5 charge marker; a minor com onent appeared at -4 net charge. The former is probably a cap 1 structure (m GpppXmYp), and the latter is probably a cap 0 (m7GpppXp). On the basis of relative labeling of the two caps with [3H]adenosine and [H]guanosine, we concluded that each cap contained at least one adenosine residue in addition to guanosine and, therefore, that cap 0 contained m7GpppAp. Cleavage of [3H]methionine-labeled viral RNA with tobacco acid pyrophosphatase released a labeled component that cochromatographed on polyethyleneimine-cellulose with m7Gp, confirming the m7GpppX linkage in the cap. These results were also seen with host polyadenylated RNA. The caps appeared in nearly equal abundance in viral polyadenylated and nonpolyadenylated RNAs. The ratio of 32p; incorporated into the cap to that incorporated into mononucleotides suggested average lengths for the polyadenylated and non-polyadenylated RNAs of 1,800 and 1,200 nucleotides, respectively.