Bathybembix bairdii, the Baird’s top shell, is reported for the first time from off the Baja California Peninsula west coast and the southwestern coast of Mexico, at depths of 710-2,093 m. Environmental conditions measured near bottom were: 2.10 to 5.81 °C, 34.40 to 34.63 (salinity), and <0.12-0.90 ml L-1 O2. The species was found mostly in muddy sediments dominated by silt (56.5-87.0%) (Baja California) or in sediments with a significant proportion of sand (63.5 to 88.5%) (southwestern Mexico). Shell height varied from 10.0 to 53.7 mm, with high proportion of large specimens in shallower water off Baja California. Densities varied from 1.75 to 207.45 org. ha-1 off Baja California and from 5.25 to 50.72 org. ha-1 in southwestern Mexico. Estimated maximum biomass occurred off southwestern Mexico (2.2 kg ha-1). Relationship between height and total weight (shell plus soft part) and between height and soft part weight were both highly correlated (R2: 0.933 and 0.928, respectively). Soft part weight varied from 29 to 62% of total weight and proportion increased with size. Mega invertebrate species associated with B. bairdii included 47% of decapod crustaceans, 35% of echinoderms, and 18% of mollusks. As a by-catch species regularly found in deep-water trawls, B. bairdii could represent a valuable resource for human consumption.Bathybembix bairdii, the Baird's top shell, is reported for the first time from off the Baja California Peninsula west coast and the southwestern coast of Mexico, at depths of 710-2,093 m. Environmental conditions measured near bottom were: 2.10 to 5.81 °C, 34.40 to 34.63 (salinity), and <0.12-0.90 ml L-1 O2. The species was found mostly in muddy sediments dominated by silt (56.5-87.0%) (Baja California) or in sediments with a significant proportion of sand (63.5 to 88.5%) (southwestern Mexico). Shell height varied from 10.0 to 53.7 mm, with high proportion of large specimens in shallower water off Baja California. Densities varied from 1.75 to 207.45 org. ha-1 off Baja California and from 5.25 to 50.72 org. ha-1 in southwestern Mexico. Estimated maximum biomass occurred off southwestern Mexico (2.2 kg ha-1). Relationship between height and total weight (shell plus soft part) and between height and soft part weight were both highly correlated (R2: 0.933 and 0.928, respectively). Soft part weight varied from 29 to 62% of total weight and proportion increased with size. Mega invertebrate species associated with B. bairdii included 47% of decapod crustaceans, 35% of echinoderms, and 18% of mollusks. As a by-catch species regularly found in deep-water trawls, B. bairdii could represent a valuable resource for human consumption.