The cytoplasmic inclusion (CI) gene of a Singapore isolate of zucchini yellow mosaic virus (ZYMV-S) was sequenced and compared with CI of 14 other potyviruses. In addition to the consensus sequence GAVGSGKST of nucleotide binding motif (NTBM) which is implicated as a membrane-binding component of the RNA helicase complex, five other conserved motifs were found. Phylogenetic trees were constructed from sequence data for the CI and the coat protein. Similar branching patterns obtained from both CI and coat protein analyses suggests that phylogenetic relationship among potyviruses can be determined using the CI. We propose that phylogenetic analysis of CI gene may be used as an alternative approach for the study of evolution within the family Potyviridae.