The complete sequence of Punta Toro virus ( Phlebovirus, Bunyaviridae) middle size (M), RNA has been determined. The RNA is 4330 nucleotides long (mol wt 1.46 × 10 6, base composition: 26.7% A, 33.6% U, 18.5% G, 21.2% C) and has 3′- and 5′-terminal sequences that, depending on the arrangement, are complementary for some 15 residues. The viral RNA codes in its viral-complementary sequence for a single primary gene product (the viral glycoprotein precursor) that is comprised of 1313 amino acids (146,376 Da) and is abundant in cysteine residues but has few potential asparagine-linked glycosylation sites. The 5′-noncoding region of the Punta Toro M viral-complementary RNA is short (16 nucleotides); the 3′-noncoding sequence is much longer (372 nucleotides). The latter is rich in short stretches of adenylate residues, like the 3′-noncoding regions of the Punta Toro S mRNA species (T. Ihara, H. Akashi, and D. H. L. Bishop, 1984, Virology 136, 293–306). No other large open reading frame has been identified in either the viral, or viral-complementary, M RNA sequences. Limited amino-terminal sequence analyses of the two viral glycoproteins have indicated the gene order and potential cleavage sites in the glycoprotein precursor. The data suggest the existence of a 30 × 10 3-Da polypeptide (designated NS M) in the glycoprotein precursor that precedes the G1 protein (i.e., gene product order: NS M-G1-G2). Examination of the sequence of the Punta Toro M gene product reveals the presence of multiple hydrophobic sequences including a 19-amino acid, carboxy-proximal, hydrophobic region (G2). This hydrophobic sequence is followed by a 13-amino acid-terminal sequence rich in charged amino acids. The size and constitution of the carboxy-terminal region is consistent with a transmembranal and anchor function for the glycoprotein in the viral envelope. Other regions of the glycoprotein precursor contain sequences of amino acids with a predominantly hydrophobic character (23, 50, and 20 amino acids in length). Their functions are unknown. The amino terminus of the G1 protein is located near the end of the 23-amino acid-long hydrophobic sequence of the presumptive precursor, the hydrophobic 50-amino acid sequence lies within G1, and the amino terminus of G2 is located in the middle of the 20-amino acid-long hydrophobic sequence. From the recently determined sequence of the M RNA gene product of Rift Valley fever virus ( Phlebovirus, Bunyaviridae; M. S. Collett, A. F. Purchio, K. Keegan, S. Frazier, W. Hayes, D. K. Anderson, M. D. Parker, C. Schmaljohn, and J. M. Dalrymple, 1985, Virology 144, 228–245) considerable homology with the Punta Toro M RNA gene product has been detected, except for the indicated NS M protein. No homology has been identified between snowshoe hare bunyavirus ( Bunyavirus, Bunyaviridae) glycoprotein precursor and that of Punta Toro virus.