Mesopelagic fishes were collected from depths of 400 to 900 m off Oregon (USA) in September 1983, October 1985 and June 1989. Species of mesopelagic fish without diel vertical migrations (non-migrators) are known to have high body-and muscle-water contents relative to epipelagic and vertically migrating mesopelagic species. This characteristic is reported to correlate with low food availability. Through histology, we show that four species of swimbladderless non-migrators (Bathylagus pacificus, B. milleri, Tactostoma macropus and Chauliodus macouni) have large deposits of gelatinous material which stain positively for acidic glycosaminoglycans (GAGs) and negatively for lipid and protein. GAGs are highly hygroscopic and are thus responsible in part for the high body-water content of these species. The material is located between myotomal (swimming) muscle cells, along the dorsal midline and surrounding the spine in all four species, and the two Bathylagus species have an additional subcutaneous layer. The subcutaneous layer of B. pacificus has very high (96%) water, low protein (3%) and low ion contents and is positively buoyant, unlike myotomal muscle (89% water, 6.7% protein, negatively buoyant). In contrast, four species of vertical migrators (B. ochotensis, Stenobrachius leucopsarus, Tarletonbeania crenularis and Diaphus theta) have no such deposits. The role of this gelatinous material as a possible buoyancy mechanism in an energy-poor habitat is discussed.