The present paper contains an account of experiments made at the Plymouth Laboratory in the summer of 1899 with the object of determining the conditions necessary for the artificial rearing of seafish larvæ through the critical stages of their development. As is well known, the majority of the attempts which have hitherto been made in this direction have proved abortive, and such success as has been obtained has been limited to the survival of a very small proportion of the original stock of fry. There was therefore ample scope for new work upon the subject, especially for experiments based on comparative methods which should aim more particularly to elucidate the habits of fish larvæ under clearly contrasted conditions. In this way only did it appear possible to make further progress in our knowledge of the physiological peculiarities of the delicate fry and of the conditions which are essential to their healthy development in confinement. The successful issue of the present experiments is entirely attributable, I think, to the fact that they were not undertaken until practically undivided attention could be given to them, which enabled me to watch long and closely the habits of individual larvæ under different conditions, and under the guidance of these observations to modify and perfect the simple apparatus employed until the mortality became reduced to reasonable proportions.