Extracellular enzymes contribute substantially to the remineralisation of organic matter in aquatic systems. It has been suggested that marine invertebrates release endogenous enzymes through activities such as feeding, but the importance of this process has not yet been fully understood. This study aimed to investigate the activity, the stability, and the origin of extracellular enzymes present in faeces and exuviae of marine crustaceans. Phosphatase activity was investigated in faeces and digestive tissues, chitinolytic activity in exuviae. 4-Methylumbelliferyl derivatives (MUF-phosphate, MUF-N-Acetyl-β-D-glucosaminide) were used to examine enzyme activity qualitatively on agarose plates, and quantitatively in microplate assays. Further, enzymes were examined using native substrate gel electrophoresis. The influence of bacterial enzymes on phosphatase activity in faeces was examined by comparing antibiotic-treated samples with untreated samples. Enzyme stability was tested over 192h using the microassay and over 72h with native substrate gel electrophoresis. Results showed that extracellular enzymes were present and active in the faeces and exuviae of all tested species. Phosphatase activity in faeces made up between 0.07% and 6.7% of the activity in digestive tissues. Native substrate gel electrophoresis showed that the activity band patterns were the same in digestive tissue extracts and faeces extracts, as well as in exuviae extracts and exoskeleton extracts. In I. balthica and P. varians, multiple phosphatase bands were present in faeces and in digestive tissue extracts. Several chitinolytic enzymes appeared in exuviae and exoskeleton extracts. For H. gammarus only one active phosphatase band and one chitinolytic band were detected. The faeces of antibiotic-treated I. balthica and H. gammarus did not vary significantly in phosphatase activity compared to untreated animals. The phosphatase activity in the faeces of antibiotic-treated P. varians was significantly reduced compared to untreated samples. The enzymes were stable for several days at natural pH and salinity and the activity band pattern did not change within 72h of incubation. Thus, bacterial contribution was estimated to be minor. Accordingly, it was concluded that marine invertebrates release endogenous extracellular enzymes through excretion and moulting and that these enzymes very likely contribute to the pool of active enzymes in aquatic environments. Therefore, these enzymes can substantially contribute to remineralisation processes and the role of aquatic invertebrates in nutrient recycling might be more important than so far estimate.