The male accessory glands of the Colorado potato beetle,Leptinotarsa decemlineata, produce an 8kDa peptide (Led-MAGP), whose primary structure and gene have been described. To investigate the possible function of this peptide in the female, we traced the passage of Led-MAGP through the male and female reproductive tracts during and after the mating process. We used immunohistochemical procedures to localize Led-MAGP employing a specific polyclonal antiserum which was raised against a recombinant Led-MAGP. Mating couples of Colorado potato beetles were interrupted after various time intervals after the initiation of mating and fixed by perfusion to immobilize the transferred male accessory gland secretions and sperm cells. Serial sections of reproductive tracts were prepared and immunolabelled. These time-lapse studies indicated that the contents of the male accessory glands are transferred during the first few minutes of mating and are deposited in the female spermathecal duct. Sperm cells are transferred later, 4–6 min after the beginning of mating. Led-MAGP is found in two forms, namely a granular, doughnut-shaped form and a homogeneously dispersed form. After deposition in the spermathecal duct, the irnmunoreactive product dissolved within 4–6 min. It is argued that the Led-MAGP crosses the duct wall and enters the hemolymph and that its function must therefore be sought outside the female reproductive tract.