Methods for assessing the occurrence and recency of mating are important for the management and conservation of exploited brachyuran crabs. Using multiparous females of the snow crab Chionoecetes opilio, we evaluated by experiment the efficacy of three indicators of recent mating: a white deposit in the spermathecae, extended mate-guarding, and fresh grasping marks on the female pereiopods. This was done by contrasting sperm counts between the left and the right spermatheca of females that were exposed to males with the right gonopod ablated, at treatment sex ratios of 20[female]:3[male ] or 50[female]:3[male ]. We expected that sperm reserves would be balanced between the two spermathecae of non-mated females and larger in the right than in the left spermatheca of mated females. Although no mating indicator was infallible, the presence of a white deposit was the most accurate because it maximized the median difference of sperm counts between the two spermathecae for the group of presumably mated females and minimized it for the group of presumably non-mated females. The use of grasping marks overestimated the mating frequency and resulted in the misclassification of a large proportion of females. Extended mate-guarding was a slightly better mating indicator than grasping marks, but it is not practical for field studies. Classification errors associated with each indicator can be explained mostly by female and male behaviours and may vary in magnitude with sociosexual context.