Crystals grow by gathering and bonding of atoms to form an ordered structure. Typically, the growth is equally probable in all crystalline directions, but sometimes it is not, as is the case of nanowire growth. Nanowire growth is explained, in most cases, by the presence of liquid metal droplets that mediate between an incoming flux of atoms and a substrate or an existing crystal nucleus, while defining the lateral dimension. Here, we report and explain a mode of nonisotropic crystal growth observed in two wurtzite semiconductors, InN and ZnO. Being of polar structure, wurtzite crystals possess a built-in internal electric field. Thermally excited charges screen the built-in electric field during growth in a nonuniform, yet symmetric, manner, causing the formation of symmetric domains of inverted polarity. These domains limit the lateral expansion of the crystal, inducing a fiber growth mode. The mechanism described here elucidates phenomena in the growth of group III nitrides on sapphire, emphasizing the need to consider the effects of built-in electric fields in the growth of polar semiconductors.