The stored-product mite Suidasia medanensis has been found in human-provided habitats in Japan. The mite uses neral as the alarm pheromone, and also secretes (Z, E)- and (E, E)-farnesal, which were isolated from an astigmatid mite for the first time. On the other hand, S. medanensis is the only mite out of fifteen species investigated so far that does not secrete volatile hydrocarbons along with the alarm pheromone, though 9-nonadecene and 6, 9-nonadecadiene were found in a hexane extract of the mite. GC-MS, 1H-NMR and 13C-NMR of the four isomers of synthetic farnesal, which were isolated by LC and preparative GLC, cleared up some confusion in the literature concerning the stereochemistry of this sesquiterpenoid.