Females of the genus Xiphophorus, which includes unsworded platyfish and sworded swordtails, share a mating preference which favours a sword despite phylogenetic evidence that the sword was not present in the evolutionary history of platyfish. A recent molecular phylogeny, however, proposes that the platyfish arose from within the swordtails. If this is the case, the preference for a sword in platyfish may be a retained ancestral preference rather than a bias that evolved before the first appearance of the sword. To determine whether or not the preference favouring a sword is an ancestral bias present before the evolution of the sword, I tested sword preferences in the sister genus, Priapella, which lacks a sword: female P. olmecae were found to prefer conspecific males with artificial swords to those without swords. These results suggest that a pre-existing bias favouring a sword arose before the divergence of these two genera, and thus before the appearance of a sword. In addition, the strength of the preference exhibited by P. olmecae females for a sword was found to vary with sword length; as the length of the sword was increased, the strength of the preference increased. Female P. olmecae, therefore, prefer males with longer swords to males with shorter swords. This increasing preference with sword length is similar to the preference of green swordtails, suggesting that the preference has a common basis in the two groups. More generally, this work further establishes the pre-existing bias model as a viable explanation for the evolution of female preferences and male traits.