1. The moulting behaviour of the brackish water prawn, Palaemonetes varians (Leach), was studied both in the laboratory and in the field during 1956. 2. Laboratory investigations showed that after hatching from June to August, the young animals completed from 15 to 25 moults before late November when moulting ceased for the winter. The second moulting season began in March when the temperature had risen to 9–10°C and continued until November. In this period males completed 10 moults and females 8 to 11 moults depending on the date of breeding. In the third and final moulting season the first moult of both sexes occurred at the time of the second moult of the second season. Only 1–2 moults were completed by the males and 2–3 moults by the females before death occurred after breeding during May–July. The intervals between the moults decreased from March to July, the warmest month investigated, and then increased from July to November. 3. Investigations based on the appearance of calcareous concretions at the time of moulting showed that moulting in the field started at the same time and continued over the same months as in the laboratory. 4. Comparison with growth measurements obtained in a previous study showed that moulting occurred without growth from March to June. During July to November growth at each moult was greater in females than in males. 5. More extensive use of the technique applied in the field investigations could give complete data on the moulting behaviour of Palaemonetes under natural conditions. The technique may also be applicable to other Crustacea.