1. Palaemonetes pugio Hobthuis and Palaemonetes vulgaris (Say) were reared in the laboratory from eggs through metamorphosis.2. The larvae of these species are very nearly identical except for a pair of chromatophores found on abdominal sternite 2 of P. pugio but lacking from P. vulgaris. The sequence of development is the same for each species.3. Individual larvae which were of the same age or which had molted the same number of times were not necessarily alike. Larvae of the same size, regardless of age or the number of molts completed, were alike. This discrepancy between age and development seems associated with the diet of the larvae.4. The molting frequency of the larvae was independent of the rate of development.5. Descriptions of a series of P. pugio larvae which illustrate the sequence of events in larval development are given. Some of these forms may be skipped.6. The concept of the crustacean larval stage has assumed a constancy of development at variance with the facts observed in rearing expe...