Assessment of their own resource-holding potential (RHP) and that of their opponent affects the behaviours individuals use in contest competition. Contestants may use their own RHP and/or the relative RHP of opponents to make decisions and may switch these two assessment tactics during fighting. In male-male contests of hermit crabs Pagurus middendorffii, we examined whether contestants switch assessment tactics and whether they assess body size or major cheliped size as an index of RHP. Males of this species show precopulatory guarding behaviour, and male-male contests often occur when males guarding females encounter intruder males. We conducted trials of dyadic contests to investigate the assessment tactics in two phases of the contest: initial and combat. Intruders gave up the contest without escalating to physical combat when they were smaller. When physical combat occurred, it lasted longer if the difference in size of the major cheliped between guarding males and intruders was small, and males with a larger major cheliped than the opponent had a higher probability of winning the contests. These results suggest that P. middendorffii uses its own body size during the initial phase of a contest and relative major cheliped size during the combat phase to assess RHP. (C) 2012 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.