Identical chemical compounds are present in the Dufour gland secretion of female Andrena bees and in the cephalic secretion of many male Nomada bees. Females of Nomada parasitize the nests of Andrena. Many Nomada species confine their attacks to a single host species. In two such host-parasite pairs, Andrena haemorrhoa-Nomada bifida and Andrena carantonica-Nomada marshamella, all-trans farnesyl hexanoate was found to be the totally dominant component in respective secretions. In two other pairs, Andrena helvola-Nomada panzeri and Andrena clarkella-Nomada leucophthalma, geranyl octanoate is the major component. This pairwise odor correspondence is discussed in relation to critical points of contact in the life cycles of host and parasite, male and female.