The size–weight relationships, percentage water, ash-free dry weight and biochemical (protein, lipid and carbohydrate) contents of the coronate jellyfish species Periphylla periphylla have been analysed. A total of 48 medusae ranging in size from 13 to 80 mm bell diameter were collected from mesopelagic depths in the eastern Gulf of Mexico. The dry mass of whole medusae ranged from 1.12 to 10.53% of wet weight (mean 5.49%), while ash-free dry weight, a proxy for organic content, varied between 25.19 and 34.89% of dry weight (mean 30.14%). Preservation in 2% glutaraldehyde resulted in shrinkage in >75% of the medusae, with preserved bell diameters 2.9% to 28.6% smaller than the original fresh bell diameters. Preservation also produced a significant adjustment to the bell diameter to wet weight relationship. With regard to biochemical content, the typical gelatinous zooplankton trend of low carbohydrate (mean 8.99 mg gDW−1), intermediate lipid (mean 20.57 mg gDW−1) and high protein (mean 63.71 mg gDW−1) was observed. Although there was a high degree of variability in biochemistry, there was no apparent trend with size.