Three SIVMne‐infected female pigtailed macaques (Macaca nemestrina)were mated with two SIV‐negative males. The females exhibited signs of SAIDS and SIVMnewas readily isolated from peripheral blood mononuclear cells (PBMC). Both males became infected with SIVMne, developed SAIDS, and died. This is the first documented case of the transmission of SIVMnebetween adult macaques housed together. Although transmission through scratching or biting cannot be ruled out, heterosexual transmission appears the most likely mode of SIVMnetransmission in this study.