Observations on the density of pelagic marine fish eggs have indicated that they are near neutral buoyancy for most of development and have a tendency to become denser towards hatching (see Russell, 1976). However, most of this information has been based on limited laboratory observations on the relative buoyancy of eggs; only a few experiments have given more precise estimates of the absolute density of eggs based on the salinity of water in which they are neutrally buoyant (e.g. Franz, 1910; Sundnes, Leivestad & Iversen, 1965). Recently, a new technique (Coombs, 1981), based on the use of a stable column of continuously graded sea-water concentration has allowed more precise and routine measurements of the density of fish eggs throughout development.