1. When Aplysia julianaPease is kept up till 6 hours in 80% sea water, it shows an increase in oxygen consumption of about 438%, as compared with the normal respiration, during the first 30 minutes. After this time it decreases rapidly at first, more slowly later. When transferred to 100% sea water after a stay of about 6 hours in diluted sea water, the oxygen consumption continues to decrease. The weight- and blood-salinity curves show a course more or less comparable to those found by Bethe in Aplysia. However, since the animals do not recover in 100% sea water after being subjected to 80% sea water for 6 hours, it is concluded that the results obtained are those from irreparably damaged animals and hence conclusions as to osmoadjustment or osmoregulation cannot be drawn. 2. The same experiments performed with animals subjected to a stay in 95% sea water, show that here there occurs a definite, be it rather weak, osmoregulation. Aplysia juliana is therefore certainly not a “pure” osmoadjustor. 3. Review of the pertinent literature shows that (weak) osmoregulation seems to be more wide-spread in marine molluscs than was thought to be.