We have studied the posterior Hox gene Nvi-Post1 expression in the early development of the polychaete Nereis virens. This is the first evidence of the posterior group Hox genes expression during the larval development of a Lophotrochozoan. The expression begins in the trochophore hyposphere at the prospective sites of larval parapodia. As the larva develops the expression weakens and finally becomes undetectable in the nectochaete stage and juvenile worm. The Nvi-Post1 expression appears to be important for larval, but not postlarval development.