The external morphology of the cypris larva of the epibiotic barnacle Octolasmis californiana Newman, 1960, found on the gills of the portunid crab Callinectes arcuatus Ordway, 1863, from Nayarit, Mexico, is described for the first time. The cyprid is elongated, about 550 μm in length; the surface of the carapace exhibits scattered pores and short setae, with prominent frontal horn pores and five pairs of lattice organs. The arrangement of the lattice organs is different from previously reported patterns. The first pair of thoracopods is slightly different from the rest, for the endopod bears five setae and the exopod six, whereas thoracopods 2–6 bear six setae on the endopod and seven on the exopod. The external morphology of the cyprid of O. californiana combines derived characters, such as the type of lattice organs, with others that might be regarded as plesiomorphic, such as the setation pattern of the thoracopods.