To elucidate phylogenetic relationships among poecilostome families 18S rDNA sequence data were generated for seven poecilostome and one cyclopoid copopods by PCR cloning and sequencing techmiques. Phylogenetic trees were constructed by maximum parsimony, neighbor joining, and maximum likelihood methods using cyclopoid sequence as an outgroup. The results from three different analyses showed that the seven poecilostome families were eiridel into two groups: Clausidiidae-Myicolidae-Synaptiphillidae-bomolochidae and Lichomologidae-Chondracanthidae-Ergasilidae. The molecular phylogenies were consistent with those from the morphological characters. Therefore, these analyses porvide further evidence for the utility of 18S rDNA sequences in addressing phylogenetic relationships among poecilostome families.