Cg-1,with 8 homologous genes,from Meloidogyne javanica is a candidate avirulence gene corresponding to tomato resistance gene Mi-1. Comparing with its homologs,Cg-1 contains a CAATGA fragment insertion. In this study, Cg-1 homologous clones from Meloidogyne enterolobii,a root-knot nematode that is able to overcome tomato Mi-1 resistance, were firstly cloned. Meanwhile,Cg-1 and its homologous clones of an M.javanica avirulent strain in our laboratory were cloned. Comparison of Cg-1 and these homologous clones from M.enterolobii and M.javanica strains showed that Cg-1 gene was inexistent in M.enterolobii that is there were only Cg-1 homologous genes which did not contain CAATGA existing in M.enterolobii. This result further indicated that Cg-1 may be an avirulence gene. Sequence analysis of Cg-1 gene predicts that the third open reading frame (ORF) encoding peptide and its structural mRNA,or both exercise biological function of Cg-1.