Phylogenetic analysis of 519 bp of LTR region sequenced from 81 strains of HTLV-I/STLV-I. Shown is one of two equivalent trees obtained via ME estimated by neighbor-joining using an heuristic search with theTamura-Nei (1993) distance measure, negative branch lengths allowed, and the tree-bisection-reconnection swapping algorithm as implemented by PAUP*. Concordant phylogenetic trees were obtained both with MP (50% majority rule of 5000 trees; length = 624; C.I. = 0.543) and ML (ln likelihood = −3443.2; 24,327 trees examined). Conditions for MP heuristic search used unordered characters of equal weight, gaps were treated as a fifth base, and trees were rooted by the midpoint method. ML tree reconstructed by PHYLIP35 (Felsenstein 1993) subroutine DNAML. Asterisks (*) denote simian virus strains of unknown geographic origin. Accession numbers for all 81 strains used in this analysis are listed athttp://rex.nci.nih.gov/RESEARCH/basic/lgd/front_page.htm.