Cyclic adenosine 3′,5′-monophosphate (cAMP) and cyclic guanosine 3′,5′-monophosphate (cGMP) contents of cultured cells ofFrankiastrains originally isolated from nodules ofAlnus sieboldiana, Myrica rubraandElaeagnus macrophyllawere measured by enzyme immunoassays (EIA).Frankiacells, cultured for 59–121 d, had cAMP contents ranging from 2.9 to 76.1 pmol mg−1protein and cGMP contents ranging from 0.9 to 5.2 pmol mg−1protein. FollowingFrankiaculture, the media contained extremely large quantities of cAMP and significant levels of cGMP. The nature of accumulation and secretion of cyclic nucleotides by slow-growingFrankiacells was comparable to that by a fast-growing actinomyceteStreptomyces lividansTK24, suggesting that secretion of cAMP byFrankiacells may occur through the cell membrane but not by cell lysis. cAMP and cGMP contents in the symbiotic nodules, leaves and roots of actinorrhizal plants and leaves of non-actinorrhizal trees were also measured. The nodules of actinorrhizal woody plants(A. sieboldiana, E. macrophylla, E. umbellata, E. pungensandM. rubra)had cAMP contents ranging from 4 to 258 pmol g−1f. wt and cGMP contents ranging from 1.1 to 5.2 pmol mg−1protein. Most leaves and some roots of actinorrhizal plants and all the leaves of non-actinorrhizal woody plants examined contained small but significant amounts of cAMP and cGMP. This is the first report of significant contents of cAMP and cGMP in culturedFrankiacells andFrankia-infected nodules. Possible roles of cyclic nucleotides as symbiotic signals are discussed.