This study investigated the effects of circadian and seasonal fluctuations on the intermediate metabolism of the freshwater crustacean Aegla platensis. Individuals were collected each month, at 6:00 h, 12:00 h and 18:00 h, between August 1999 and June 2002 in the Municipality of Taquara, Rio Grande do Sul, Brazil (29 degrees 30'0.2''S and 50 degrees 46'50''W). Anomuran crabs were separated by sex, and samples of haemolymph, hepatopancreas, abdominal muscle, and gills were collected for quantification of glucose, proteins, lipids, triglycerides, and glycogen. No variations were observed in the analyzed metabolic parameters for the different tissues during the different times of day, either for males or females. However, the storage and use of the energetic substrates analyzed did fluctuate seasonally. These results suggest an increase in energy demand, possibly for the production of gametes during summer, incubation and egg laying during autumn and winter, and parental care during spring and summer, respectively. A similar pattern has been observed for other crustaceans.