Four specimens of the sea cucumber Ypsilocucumis californiae Massin Hendrickx, 2011 were obtained during sampling operations off western Mexico. These specimens permit identification of this species as a member of the deep-water holothuroid community off the west coast of the Baja California Peninsula. Previous records correspond to four locations (including the type locality) in the Gulf of California, where eight specimens were collected. SEM ossicles images are provided for the first time and new ecological data associated with the presence of this species are available: temperature, 5.34‒8.38 °C; dissolved oxygen, 0.15‒0.28 ml O2/l and salinity, 34.42‒34.51 ups. The specimens were present in a wide variety of sediments with an organic carbon content of 3.18‒5.20 mg C/g (5.47‒8.95 % organic matter). Density values indicated low abundance of this species in the area (2.63‒3.94 orgs/ha). Records presented here were in a depth range from 540 to 776 m, which corresponds to the lower limit of the Oxygen Minimum Zone of the eastern Pacific. Additional records are provided for the West Atlantic Ypsilocucumis asperrima (Théel, 1886) and a key to the species of Ypsilocucumis is provided.
Keywords: Echinodermata, Oxygen Minimum Zone, Deep-sea waters, Baja California Peninsula, Ypsilocucumis asperrima, diagnostic key.