3-Isopentyl-2,5-dimethyl-, and the (Z)- and (E)-isomers of 2,5-dimethyl-3-styryl-pyrazine, have been fully characterized from the Argentine ant, I. humilis. A facile conversion of the (E)- into the (Z)-styryl derivative is noted. These pyrazines comprise 70 and 200 p.p.m, of the body weight of the insect. Mass spectrometric data suggests a further pyrazine, the 2,5-dimethyl-3-propyl derivative, is present at approximately 5 p.p.m.