1. 1. Embryos and larvae of the asteroid Echinaster spinulosus were exposed to high salinity stress at various stages during early development. 2. 2. Highest percentages of three-rayed (9.7%) and four-rayed (29%) individuals occurred when individuals which had developed for 48 hr (appearance of pre-oral lobe) at ambient salinity (30%o) were exposed to high salinity (39%o). 3. 3. The percentage of ray-number aberrancies increased with increasing salinity. 4. 4. The ontogenetic events associated with the formation of the hydrocoelic rudiments at the pre-oral lobe stage may be sensitive to salinity and influence the development of ray number. 5. 5. The ability to induce variations of ray number in asteroids with salinity stress may yield an experimental approach for the determination of the adaptive significance of ray number in asteroids. © 1986.