Crabs of the genusTrapezia, obligate symbionts of reef corals, typically occur as heterosexual pairs, with a single pair of a given species per coral colony. Pair formation, and the effects of size on pairing, were studied inT. intermediaandT. digitalisin Hawaii, inT. tigrina, T. rufopunctata, and T. dentataat Enewetak Atoll, Marshall Islands, and inT. corallinaat Panama. When a male and female of the same species were placed on separate corals, either the male or the female usually moved to the colony inhabited by the other crab, resulting in the formation of a pair. Both males and females paired with the larger of two prospective mates significantly more often than expected on the basis of random pairing; the “preference” for large mates did not vary with the relative sizes of the crabs. The distribution of injuries which occurred in the experiments indicated that there was aggressive competition for mates between members of the same sex. While the experimental results can be explained by intra‐sexual competition for mates, the hypothesis that crabs exercise choice for large mates cannot be ruled out, and possible adaptive advantages of mate choice are discussed.