We propose to perform small-angle neutron scattering investigations of the vortex lattice (VL) in underdoped, twinfree, and highly CuO-chain ordered samples of the High-Tc superconductor YBa(2)Cu(3)O(6+x). Our previous studies have been performed on stoichiometric and twinfree YBa2Cu3O7, which is close to optimal doping. This new study aims to examine the VL on two samples of varying underdoping, for doping levels where field-induced charge-density-wave (CDW) order has been reported.