The biochemical composition of the invasive ctenophore, Mnemiopsis leidyi from the southern Caspian Sea was analysed in terms of total fractions of protein, lipid, ninhydrin-positive substances (NPS), polysaccharides ( PS) and low molecular-weight carbohydrates (LMC). The sum of organic substances per gram dry weight was 84.2 mg g(-1). Protein was the major organic fraction followed by lipid and NPS. The NPS/protein and LNC/PS ratio were 0.45 and 0.50 respectively. Starvation of M. leidyi for a 5-day period resulted in a 32.7% reduction of total organic content. These data will be important in evaluating the feeding condition of M. leidyi specimens from the field.