Three planes are discerned during ultrasonographic screening of the insula in parasagittal view: opercular, insular, and fissural. Six newborn infants with normal brain anatomy, including two each of 28, 34, and 40 weeks' gestation, were selected for a description of the evolution of these parasagittal planes. Opercularization of the insula begins to be detected on a sonogram at about the 24th gestational week and progresses cranially. On coronal section the insular space forms a shallow groove at 24 weeks, becoming a slit at 28 weeks that grows longer and develops branches after 32 weeks. At 28 weeks the ascending anterior branch of the circular groove and the lateral fissure at the bottom of the insula are clearly seen in parasagittal section. Subsequent change consists of undulation and bifurcation of the lateral fissure, together with elongation of the anterior margin. Secondary gyri become visible in the insular dome between 28 and 34 weeks, forming short anterior and long posterior insular gyri. Five term newborn infants with perisylvian polymicrogyria were observed with both neonatal ultrasound and magnetic resonance imaging: two of unknown cause, and one each due to cytomegalovirus, bifunctional peroxisomal protein deficiency, and monozygous twinning. With polymicrogyria, on parasagittal sonographic examination at fissural level the anterior margin of the circular groove is a short rudiment. In insular view, only rudimentary sulci ascend from the lateral fissure; short and long gyri are not seen. In opercular view, no secondary branches are seen from the lateral fissure, an image akin to that seen at 28 weeks. A complete insular triangle is not recognized in any of these sections. A deep abnormal sulcus may prolong the lateral fissure when schizencephaly is associated with polymicrogyria. The insula in glutaric aciduria type II has no secondary gyri at term and is reduced to a simple and small triangle. Not all instances of polymicrogyria lead to macroscopically recognizable alteration of insular gyri.