The effects of 1 mM solutions of FMRFamide-like peptides (FLPs) on the behaviours of Heterodera glycines and Meloidogyne incognita infective juveniles (J2) were examined in vitro. Seven FLPs, representing products of five flp genes and comprising a variety of amino acid sequences, were tested for their effects on J2 head movement frequency. Distinct differences in species responses were observed. KHEYLRFa and KSAYMRFa caused increased head movement frequencies in each species. In H. glycines KHEYLRFa was 2.9-fold more potent than KSAYMRFa. In M. incognita the potencies were equal. KHEYLRFa and KSAYMRFa each stimulated H. glycines J2 more effectively than M. incognita J2. However, two additional FLPs, AQTFVRFa and SAPYDPNFLRFa, were stimulatory in M. incognita but not in H. glycines. KPNFIRFa, KPNFLRFa and RNSSPLGTMRFa had no effect on either species. Substitution of d-amino acids at any position in KHEYLRFa resulted in decreased stimulation of head movement relative to the native peptide in each species, but all of the d-amino acid KHEYLRFa analogues were stimulatory relative to untreated controls. d-amino acid substitutions in KSAYMRFa eliminated stimulatory activity in M. incognita by all analogues except dKSAYMRFa. In H. glycines, only KSdAYMRFa and KSAYMdRFa were not stimulatory, and KSAYdMRFa stimulated equal to the native peptide. The remaining four analogues each stimulated relative to controls but below the native peptide level. Analysis of the head movement behaviour of large numbers of J2 of each species demonstrated that behaviours are quite stable and responses to FLP treatment are highly predictable.