When a larva of the flour beetle Tribolium freemani (Hinton) was isolated (one larva per vial), the larva pupated within 6 days, whereas at higher larval densities (two and four larvae per vial) pupation was delayed. Meanwhile, there was an increase of whole-body octopamine (OA) levels, measured using radioenzymatic assay, during the last part of intermoult before pupation in the three groups of insects. The onset of the OA increase was delayed by increased larval density. At larval densities of 1 and 2 larvae per vial the pupation coincided with the OA decrease, whereas at increasing larval density (four larvae per vial) the pupation started after the onset of the OA decline. Juvenile hormone (JH) I reduced the pupation of an isolated larva of T. freemani. Chlordimeform and other OA agonists and precocene II accelerated the pupation of crowded larvae, which was antagonized by JH I. JH esterase (JHE) of larvae treated with an OA agonist was higher than that of control larvae. Hence, OA may be responsible for stimulated pupation of isolated T. freemani, possibly by activating JHE.