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.