Neral [3,7-dimethyl-(Z)-2,6-octadienal] was identified to be the female sex pheromone of the mite Histiogaster sp. (Acari: Acaridae). The species originated from a fungal Trichoderma harzianum culture in Japan. The compound aroused males to mount females. Although the compound was detected in both sexes (29.4±8.9 ng in females and 5.7±1.9 ng in males), the behavior observation suggested that males could distinguish females from males. Synthetic neral indicated the activity at 0.1-1 ng doses. On the other hand, geranial [3,7-dimethyl-(E)-2,6-octadienal, the geometrical isomer of neral at the 2-C position], did not sexually stimulate males. Males could, therefore, discriminate between neral and geranial. Escaping behavior was observed upon exposure of neral at 100 ng and of geranial at 10-100 ng doses.