Introduction: Megacystis microcolon intestinal hypoperistalsis syndrome (MMIHS) is characterized by bladder distension without urinary tract obstruction, decreased or absent intestinal peristalsis and microcolon. Although the definitive cause remains unknown, changes in the ACTG2 gene are thought to be responsible for the intestinal and bladder hypoperistalsis. Case report. This female newborn with MMIHS had a c.532C > A /p.Arg178Ser heterozygous de novo mutation detected in the ACTG2 gene. Normal immature ganglion cells, normal calretinin punctate positivity, maintence of smooth muscle actin immunoreactivity, and decreased numbers of interstitial cells of Cajal(ICCs) were detected. Conclusion: This previously unreported c.532C >A /p.Arg178Ser heterozygous de novo mutation in the ACTG2 gene may lead to a severe form of MMIHS.