Structural changes of the vitelline envelopes during oogenesis of a polychaete, Nectoneanthes oxypoda, were examined with a scanning electron micrscope. Oocytes grow in the same coelomic fluid to the final stage, but the surface appears to change in the structure during oogenesis. Projections, which were identified to be microvilli, change in shape, number and size. Short microvilli, which cover the surface of oocyte of $33{mu}m$ diameter densely, grow in length, reaching a maximum at the stage of $73{mu}m$. The number of microvilli increases with the stages of oogenesis, reaching a plateau at the stage of $82{mu}m$. The observations suggest that control of material transport including yolk precursor proteins may be correlated with the structural changes in the microvilli.