AbstractThe free‐living nematodePanagrellus silusiaecan be grown synchronously through the postpartum period of its life cycle. This period is divided into four phases which in sequence are called the L2, L3, L4 and adult stages. Homogenates from each stage were analyzed electrophoretically in order to discover whether protein changes occur during postembryonic development. Protein patterns in acrylamide gels from L2, L3, L4 and adult stages show no differences in the number of bands (16), although there are differences in the intensity of certain bands at various stages.Definite stage specific patterns were observed when enzyme activity was assayed histochemically. L4 larvae show five sites of lactate dehydrogenase activity in polyacrylamide gel; three of these are found in each of the other stages. L3 larval extracts have five sites of malate dehydrogenase activity; L2 larvae and adults share one of these and L4 larvae have two in common. L4 and adult nematodes each have one MDH type not present in any other stage. L4 larvae produce six sites of α‐naphthyl acetate esterase activity; four of these bands are present in each of the other stages. Both acid and alkaline phosphatase occur as single molecular forms in each postpartum stage.It is concluded that during maturation a precise regulatory program operates to control the sequential appearance of specific enzymatic prot