Ephemera simulans maintains an oxygen consumption in a suitable substratum of approximately 0.23 cc/g/hr from at least February, and probably before, through approximately the middle of April at which time it rises to around 0.37 cc/g/hr. Such data indicate quite clearly that a rise in metabolic rate occurs in the spring in this burrowing mayfly. A surge of growth and gonad development in April and May is here suggested as the factor most likely responsible for the metabolic increase. Observation of successive collections of nymphs at this time of year makes obvious the rapid development.