The trail pheromone of the antTetramorium caespitum L. is a 70∶30 mixture of 2,5-dimethylpyrazine and 3-ethyl-2, 5-dimethylpyrazine. The average total amount of the two pyrazines present in the poison vesicle was found to be 3.9 ng per ant, of which 2.7 ± 0.4 ng is 2,5-dimethylpyrazine and 1.15 ±0.25 ng is 3-ethyl-2,5-dimethylpyrazine. The pyrazines constitute only 0.03% of the volume of the poison vesicle but account for the whole of the trail-following activity. A 70∶30 mixture of the respective pyrazines evoked the highest activity in artificial trail-following tests.