The principal aim of this study was to test if biochemical and physiological data on decapod crustacean larvae from laboratory experiments can be used for the interpretation of comparable field data. Different parameters measured under identical conditions in laboratory-reared and field-collected larvae of Carcinus maenas were compared with each other. The parameters used were: dry weight (W), elemental composition (C, N), protein and lipid content, activities of digestive enzymes (amylase, trypsin), respiration, and ammonia excretion. Effects of different laboratory diets (Artemia sp., diatoms and no food, representing near optimal, suboptimal and starvation conditions respectively) were evaluated as a possible base for the interpretation of field data. Field data were in a similar range to laboratory data, with the exception of dry weight and protein:lipid ratio. Comparison of pooled data sets for total larval development showed lower nitrogen and higher protein contents (in % of W) in field samples than in laboratory cultures, resulting in different N:protein relations. Best agreement in these parameters was found between field data and laboratory data from phytoplankton-fed or starved larvae, whereas exclusively Artemia-fed larvae showed higher percentages of C, N, and lipid. Larval W in stages later than zoea Il was consistently higher in the field. The protein content (in mug ind.-1) was higher in the field as compared with laboratory cultures, resulting in a lower lipid:protein ratio. Comparison of lipid data and digestive enzyme activities under different nutritional conditions in the laboratory and in field samples shows that nutrition is limited in the field, and phytoplankton may constitute a major component of the natural diet of C. maenas larvae. This conclusion is confirmed by direct evidence from gut fluorescence and scanning electron microscopic (SEM) examination of faecal pellets. Our study suggests that laboratory data on the physiology and biochemistry of decapod larvae may be a useful tool for the evaluation of field data.