A lack of complex and hybrid types of N-glycans in mice is embryonically lethal due to neural tube maldevelopment. N-acetylglucosaminyltransferase-I (GnT-I; Mgat1) catalyzes a required step for converting oligomannose N-glycans into hybrid and complex N-glycans. Unlike mice, zebrafish have two Mgat1a/b genes. Herein, CRISPR/Cas9 technology was used to knockdown GnT-Ib activity in zebrafish, referred to as Mgat1b–/–, to examine the impact of a decrease in complex types of N-glycans on survival and development, and sensory and motor functions. […] the results reveal that reducing the amount of complex-type N-glycans is unfavorable for zebrafish survival and development. Moreover, our results support a better understanding of human congenital disorders of glycosylation.