The biodiversity of the deep-sea of the Gulf of California has received some attention during the last two decades. Preliminary observations have revealed a high diversity of benthic and hyperbenthic copepods. Copepods of the genus Rhizothrix were found in sediment samples taken during the Talud X cruise in the Guaymas Basin. These specimens belong to a new species, Rhizothrix longiseta. The new species is related to R. quadriseta Wells, 1967 from Inhaca Island, Mozambique by the armature complement of the third exopodal segment of the second, third and fourth swimming legs, and of the second endopodal segment of the third swimming leg. The females of these two species can be distinguished by the armature complement of the second endopodal segment of the first and second swimming legs, by the relative length of the inner seta on the third exopodal segment of the second, third and fourth swimming legs, by the shape and number of setae on the exopod of the fifth leg, by the length:width ratio of the caudal rami, and by the number of segments of the antennule. This is the first record of the genus Rhizothrix from the deep-sea down to 1570 m depth, and is also the first record of the genus for Mexico. Additionally, R. reducta noodti Galhano, 1970 is given full species rank as R. noodti.