The insects, Scatella stagnalis, for the present study were collected from the Marine Biological Laboratory at Plymouth at the kind suggestion of Dr. Allen, F.R.S., the Director of the Laboratory, during my short stay there in the summer of 1929. So far as I can see, no work on the early stages of the genus Scatella is available. This led me to put on record the present account of the different pre-imaginal stages of the insect Scatella stagnalis in the following pages. I have described here only those stages that are exhibited in my hurried collection which may not be complete. A study of its complete biology would be of considerable interest; this, however, requires a constant watch of the species at the spot for a fairly long period.