A histochemical technique, applicable to intact euphausiids, has been used in the laboratory to demonstrate variations in the level of subcuticular acid phosphatase activity during the molting cycle of Thysanoessa raschii; it shows that there is a marked increase in such activity during the 24 hours in which ecdysis occurs. We suggest that the technique could form a useful tool in future studies on euphausiid biology since it would allow, for example, assessments of the numbers of individuals in any one collection or population that is passing through this particular 24-hour period at the time of sampling.