The optimal conditions for storing and assaying mesozooplankton aspartate transcarbamylase (ATCase) (EC 2.1.3.1.) are denned in order to enable the specific activity of this enzyme to be used as an index of mesozooplankton productivity. ATCase activity was found to be stable for more than 2 weeks when stored in liquid nitrogen, with a negligible loss of activity, whatever the state of frozen material (cell-free homogenate or whole cells). As an alternative,-20°C,-90°C freezers and dry ice (-80°C) can give more flexibility for storing and transporting material, provided initial freezing has been carried out at-196°C. ATCase specific activity was stable over a large range of total protein concentrations (2-55 mg ml"') for most mesozooplankton samples, and the enzyme activity was linear between 0.8 and 80 nmol carbamyl aspartate min~' mg~' of total protein. The optimal incubation time (30 min), temperature (35°C) and pH range (9-9.5) for assay of ATCase were similar for Calanus helgolandicus (Crustacea: Copepoda) (G.O.Sars) and three mesozooplankton assemblages sampled off Plymouth (UK) in May, August and November. However, the K m values for both substrates were variable: from 3.9 to 79.1 for aspartate, and from 0.83 to 3.46 for carbamyl phosphate. Such variations are likely to be due to the presence of different ATCase catalytic potential, rather than changes in ATCase assemblage.