The td, nrdB, and sunY introns of bacteriophage T4 each contain a long open reading frame (ORF). These ORFs are preceded by functional T4 late promoters and, in the case of the nrdB intron ORF, a functional middle promoter. Expression of phage-encoded intron ORF-lacZ fusions indicates that these T4 genes are highly regulated. The lack of translation of these ORFs from early pre-mRNAs can be accounted for by the presence of secondary structures that are absent from the late RNAs. Because translation of the intron ORFs could disrupt core structural elements required for pre-mRNA splicing, such regulation may be necessary to allow expression of the genes in which they reside.