1. Tissue levels of adenosine 3', 5'-cyclic monophosphate (c-AMP) were determined by a protein binding assay in mixed populations of the free living nematode Panagrellus redivivus. 2. The values were 2.6 pmoles/mg dry weight (range 1.3-3.4 pmoles). 3. The identity of the c-AMP assayed was confirmed by both anion exchange and TLC as well as enzymatic degradation.