A behavioural assay based on a ‘homosexual’ response in males has revealed that female Harmothoe imbricata release a substance that induces pair formation. Extracts from unfertilized spawned oocytes (diameter approximately 120 μm), but not sub-mature oocytes (90 μm diameter) removed directly from the coelomic cavity, have significant pair formation-inducing activity in comparison to the control. Preliminary results indicate that the substance is stable under standard purification techniques and larger than 10 kDa, as well as being heat and trypsin insensitive.