Ultrastructural study of the midgut of Calanoid Copepods revealed the presence of several cell types in all species. In a previous report we described and assigned a function to each of these cell types. In order to affirm the validity of those assignments we undertook an investigation of enzymatic activity especially of phosphatase and arylsulphatase. By cytochemical methods, alkaline phosphatase activity was detected in R-, R'-D- and B-cells, with labelling being observed on the apical plasmic membrane level in all four, and in B-cells on the pinocytotic vesicle membranes. Acid phosphatase and aryl-sulphatase activities were only detectable in B-cells; the most frequently labelled structures were located in the vacuolar system, dictyosomes and Golgi vesicles, although Golgi structures occasionally reacted to acid phosphatase. Nome of the dense bodies observed in B-cells reacted to arylsulphatase. Similarly they were unevenly labelled during acid phosphatase tests. Hence it may be assumed that dense bodies are not involved in hydrolases. It is possible that these enzymes originated from vesicles generated by the Golgi saccules surrounding and joined to the vacuoles, thus bypassing the lysosome I stage.