ABSTRACTIndirect calorimetry and the chromic oxide indicator method were employed to investigate the digestibility and assimilation of nutrients by cultured lobsters (Homarussp.) growing on experimental diets. Starved and fed lobsters were compared by observing food and oxygen consumption and fecal and urinary excretions in a closed, recirculating metabolic chamber. Apparent digestible and metabolizable energy and nitrogen were determined for 2 diets, which differed in the type and level of protein and carbohydrate. The apparent digestibility of diet 79F, a purified ration containing 46.8% crude protein, was 68.8% on an as‐fed basis, with an apparent digestibility of energy of 88.1%. For diet SM25B, which contained 61.1% crude protein including shrimp meal, apparent digestibility was 66.2% as‐fed and the apparent digestibility of energy was 91.3%. Metabolizable energy values of 4.59 and 4.65 kcal/g dry weight were obtained for diets 79F and SM25B respectively. Growth on the two diets was similar but lobsters fed 79F consumed and excreted significantly more than those fed SM25B. For both diets protein and lipid were better than 90% digested, but the carbohydrate fraction was only about 65% digestible. The apparent higher nutritive value of SM25B for lobsters may be related to protein quality and improved efficiency of utilizat