Larval competition and allometry in the stag beetle Dorcus rectus (Motschulsky) were investigated in the laboratory and the field. Allometry indicated three larval groups of different head capsule widths and body masses, representing three instars. A laboratory experiment, in which two larvae were placed on milled decaying wood in test tubes for two weeks, showed that cannibalism occurred in the first and second instars. Cannibals tended to have larger head capsules than their victims. Cannibalizing larvae gained more body mass than non-cannibals. The carbon/nitrogen ratio of decaying wood was much higher than that of larvae, explaining an increased body mass following cannibalism. Sixty-two percent of surviving second instars was wounded by their opponents. When cannibalism did not occur by second instars, large-headed larvae grew but the growth of small-headed larvae was restricted, suggesting strong interference. However, a field study suggested low rates of interference competition between larvae.