Seasonal variations of size-frequency distribution, sex ratio, and percentage of egg-carrying females andjuveniles in a population of J. falcata inhabiting jetties at Helgoland Harbour are described. Reproductionoccurs all the year round, but 2 maxima and thus, 2 main generations per year can be observed: a weaker onein winter, and a more pronounced one in late spring/early summer. Biochemical composition and energycontents of J. falcata show only in part a seasonal cycle. There is an inverse relationship between the proteinand lipid fractions, whereas the former is negatively, the latter positively correlated with the amount ofsuspended food in the water. Protein, carbohydrates, lipid and the weight-specific energy equivalent show adecreasing trend with increasing size of the amphipods, while chitin significantly reveals an opposite trend.