1. Thirty-three discinid larvae were obtained one meter below the water surface in the daytime in the shallow part of the Johore Strait, north of Singapore. 2. The oldest among them, 462 µ in longitudinal diameter, resembled the "Mueller's larva" discovered by Mueller and referred to Pelagodiscus atlanticus (King) by many subsequent authors. 3. The youngest among them, 108 µ long and 115 µ wide in dorsal shell, resembled Simroth's smaller discinid larva of 220 µ diameter in many essential features. 4. The larvae in this study showed a gradual transition in many morphological characters, indicating that they were a series of developmental stages in the ontogeny of a single discinid species. 5. The two bunches of long embryonic setae were gradually replaced by the curved early larval setae in the medium-sized specimens. Towards the close of the larval stage the third kind of setae appeared from the mantle border. 6. The dorsal shell valve maintained its oval shape during the entire larval stage. The posterior edge of the ventral one, however, gradually changed from convex to concave, passing through a stage when it was almost a straight line. 7. The statocysts and the pedicle appeared in older larvae. 8. The number of cirri remained constant throughout the larval stage. 9. The larvae were compared with the discinid larvae previously recorded. Their developmental changes and the breeding season of discinids were discussed.