Structures for all sequences containing less than 1790 nucleotides in the 2600 nucleotide late region of the SV40 virus have been computed and saved on magnetic tape. Previously the longest sequence whose secondary structure was calculated in a single computer run contained 950 nucleotides. In the past, analysis of long molecules required nisnerous repeated, partially overlapping computations on much shorter segments. The structure obtained for the late half of the SV40 is Y- shaped with two unequal arms. It has 52 short hairpins. Two long range interactions between nucleotides near 650 and 1350 and between 1450 and 2450 appear to play an important role. The first is within the l6S intron; the second is in the 3′ exon. The 5′ and 3′ ends of the molecule are close to each other and are found in the major elongated stem in the vicinity of the fork.