none The effects of parasitism by A. glomeratus on the hemolymph juvenile hormone (JH) titers of P. brassicae during the last larval instar were determined using the Galleria bioassay. Depending on the time of parasitization, a significant increase of the JH titer could be observed when the parasites molted from the 1st to the 2nd larval instar. As neck-ligatured, parasitized Pieris larvae showed a similar increase of the JH titer at this time the parasite larvae themselves are concluded to be responsible for the elevation of the titer by delivering JH produced during their ecdysis into the host's hemolyph. This is supported by histological results from the corpora allata of parasitized and unparasitized caterpillars at the 1st and 3rd day of the last larval instar, indicating no differences in its secretory activity. The prothoracic glands of parasitized host larvae, however, appear significantly smaller than those of comparable unparasitized ones in the middle of the last instar. A reduced secretory activity at this time, which is assumed from their decreased size, combined with an elevated JH titer may explain why parasitized larvae fail to molt at the end of their larval development.