We report here on the first observations and capture of breeding populations of the largemouth blenny, Labrisomus xanti Gill, 1 860 (Family Labrisornidae) in California marine waters. We also provide a list of those members of the closely related families Labrisornidae, Clinidae, and Chaenopsidae that are found off California with a key to these species.The first observation of this species in California waters occurred on 15 July 2015, when Callie Mack and Tara Howell observed one individual in about 5 m of water in front of the Marine Room Restaurant (La Jolla Shores) (32°5rN, 117°16*W) among low lying rocks. Ms. Mack described the fish as having "a blenny-like profile, about 6 or 7 inches long (the length of my hand), bright red and covered with many small blue spots, and a partial bright blue ring at the base of each eye. It was perched in a rock crevice on its red pelvic fins. Pectoral fins were red shading to yellow. Dorsal fin was also bright red with blue spots on the foredorsal [sic] part. It had 3 rows of small for-like (or eyelash-like) cirri, also covered with blue spots, on the top of its head, one set just below each eye, and 2 rows above the eyes on the forehead. It was either curious or territorial (probably the latter); kept coming out of its crevice, sitting in the open for a few moments, then going back in."Ms. Mack and Ms. Howell returned to the same general area on 19 July 2015 and observed at least four different individuals, two on each dive and all at a bottom depth of 3-4 m. On this occasion she provided us with a number of images of one of the individuals that was red and she described it as quite territorial as it chased off a California sheephead and lunged at Ms. Howell's video camera. From Ms. Mack's initial description, and her subsequent photographs, we determined that these fish were most likely the largemouth blenny, Labrisomus xanti and that the red individuals were breeding males (Thomson et al. 2000),Additional observations were made of this species at the same general site on 11 and 19 August 2015 by Rogei Uziin who observed at least five unique individuals. Similar to previous sightings, he observed the fish at depth of 3-4 m on low-lying rocks. However on these days, Mr. Uzun observed what was likely mating behavior and nest guarding. On both 1 1 and 1 9 August, he observed, photographed, and video recorded a bright red individual (Fig. 1) courting and apparently fertilizing the eggs of two drabber individuals (Fig. 2). This male then guarded and aerated the apparent eggs. Similar courtship and parental behaviors are known for other species of the genus Labrisomus (Gibran et al. 2004).Janna Nichols made the first observation of this species known to us away from the